IMac 27" (Late 2012) Freezing and Crashing

Question

I purchased an ex-demo iMac 27" (Late 2012 - 3.4Ghz, 3TB Fusion Drive, Nvidia 680MX 2GB) that was upgraded to 32GB Ram last July.
Since I've recieved it it has intemittantly locked up, occasionally you get the your computers been restarted screen but mostly it just freezes completely.  There's no pattern it can be at the login screen, on the desktop with no software running or when any software I use.  There's no particular thing that seems to cause it.
I have run the Apple Hardware Test in Extended Mode Multiple Times and it reports no issues.  I've run MemTest Continuosly for 24 hours and it reports no issue.
I've done clean installs of OSX multiple times (both Mavericks and Mountain Lion) and its had no effect.  Even with a barebones OSX install it still freezes so I've ruled out software.
I've switched the RAM to rule out memory - still freezes.
I've used both WiFi and and Ethernet to rule out network - still freezes.
I've removed the Fusion Drive Logical Volume and installed OSX to the SSD and then to the HDD to rule out hard disks.  This included zeroing the hard drive to ensure there was no bad sectors etc.  This didn't make any difference.
I contacted Apple who told me to take it to an Apple Store or Apple Authorized Service Provider.  I took it in to my local AASP they've told me they can't find anything wrong - everything works fine for them.
I'm wondering whether the issues are environmental.  I've seen a few reports that "Dirty Power" can cause issues, has anyone had any similar experiences that have been solved by a UPS?
Failing that I'm at a loss at what to do.  The iMac is unusable as it freezes every few hours.
Anyone have any ideas of what else I could try?
Thanks,
James

Answer 1

1. This procedure is a diagnostic test. It changes nothing, for better or worse, and therefore will not, in itself, solve your problem.
2. If you don't already have a current backup, back up all data before doing anything else. The backup is necessary on general principle, not because of anything in the test procedure. There are ways to back up a computer that isn't fully functional. Ask if you need guidance.
3. Below are instructions to run a UNIX shell script, a type of program. All it does is to gather information about the state of your computer. That information goes nowhere unless you choose to share it on this page. However, you should be cautious about running any kind of program (not just a shell script) at the request of a stranger on a public message board. If you have doubts, search this site for other discussions in which this procedure has been followed without any report of ill effects. If you can't satisfy yourself that the instructions are safe, don't follow them.
Here's a summary of what you need to do: Copy a line of text from this web page into the window of another application. Wait for the script to run. It usually takes a couple of minutes. Then paste the results, which will have been copied automatically, back into a reply on this page. The sequence is: copy, paste, wait, paste again. Details follow.
4. You may have started the computer in "safe" mode. Preferably, these steps should be taken in “normal” mode. If the system is now in safe mode and works well enough in normal mode to run the test, restart as usual. If you can only test in safe mode, do that.
5. If you have more than one user, and the one affected by the problem is not an administrator, then please run the test twice: once while logged in as the affected user, and once as an administrator. The results may be different. The user that is created automatically on a new computer when you start it for the first time is an administrator. If you can't log in as an administrator, test as the affected user. Most personal Macs have only one user, and in that case this section doesn’t apply.
6. The script is a single long line, all of which must be selected. You can accomplish this easily by triple-clicking  anywhere in the line. The whole line will highlight, though you may not see all of it in your browser, and you can then copy it. If you try to select the line by dragging across the part you can see, you won't get all of it.
Triple-click anywhere in the line of text below on this page to select it:
PATH=/usr/bin:/bin:/usr/sbin:/sbin; clear; Fb='%s\n\t(%s)\n'; Fm='\n%s:\n\n%s\n'; Fs='\n%s: %s\n'; Fu='user %s%%, system %s%%'; PB="/usr/libexec/PlistBuddy -c Print"; A () { [[ a -eq 0 ]]; }; M () { find -L "$d" -type f | while read f; do file -b "$f" | egrep -q XML\|exec && echo $f; done; }; Pc () { o=`egrep -v '^[[:blank:]]*($|#)' "$2"`; Pm "$1"; }; Pm () { [[ "$o" ]] && o=`sed 's/^ */   /' <<< "$o"` && printf "$Fm" "$1" "$o"; }; Pp () { o=`$PB "$2" | awk -F'= ' \/$3'/{print $2}'`; Pm "$1"; }; Ps () { o=`echo $o`; [[ ! "$o" =~ ^0?$ ]] && printf "$Fs" "$1" "$o"; }; R () { o=; [[ r -eq 0 ]]; }; SP () { system_profiler SP${1}DataType; }; id | grep -qw '80(admin)'; a=$?; A && sudo true; r=$?; t=`date +%s`; clear; { A || echo $'No admin access\n'; A && ! R && echo $'No root access\n'; SP Software | sed '8!d;s/^ *//'; o=`SP Hardware | awk '/Mem/{print $2}'`; o=$((o<4?o:0)); Ps "Memory (GB)"; o=`SP Diagnostics | sed '5,6!d'`; [[ "$o" =~ Pass ]] || Pm "POST"; o=`SP USB | awk '/:$/{d[++n]=$0}; /M.*: /{split($0,a,":"); m[n]=a[2]} END {for (i in m) {if (m[i] !~ /^ Apple/) print d[i] m[i]}}'`; Pm "USB"; o=`pmset -g therm | sed 's/^.*CP/CP/'`; egrep -q 'No th|pms' <<< "$o" && o=; Pm "Thermal conditions"; o=`pmset -g sysload | grep -v :`; grep -q '= [^GO]' <<< "$o" || o=; Pm "System load advisory"; o=`nvram boot-args | awk '{$1=""; print}'`; Ps "boot-args"; d=(/ ""); D=(System User); for i in 0 1; do o=`cd ${d[$i]}L*/L*/Dia* && ls | awk -F_ '!/ag$/{$NF=a[split($NF,a,".")]; print}' | tail`; Pm "${D[$i]} diagnostics"; done; o=`syslog -F bsd -k Sender kernel -k Message CReq 'GPU |hfs: Ru|I/O e|n Cause: -|NVDA\(|pagin|SATA W|ssert|timed? ?o' | tail -n25 | awk '/:/{$4=""; $5=""; print}'`; Pm "Kernel messages"; o=`df -m / | awk 'NR==2 {print $4}'`; o=$((o<5120?o:0)); Ps "Free space (MiB)"; o=$(($(vm_stat | awk '/eo/{sub("\\.",""); print $2}')/256)); o=$((o>=1024?o:0)); Ps "Pageouts (MiB)"; s=( `sar -u 1 10 | sed '$!d'` ); [[ s[4] -lt 85 ]] && o=`printf "$Fu" ${s[1]} ${s[3]}` || o=; Ps "Total CPU usage" && { s=(`ps acrx -o comm,ruid,%cpu | sed '2!d'`); o=${s[2]}%; Ps "CPU usage by process \"$s\" with UID ${s[1]}"; }; s=(`top -R -l1 -n1 -o prt -stats command,uid,prt | sed '$!d'`); s[2]=${s[2]%[+-]}; o=$((s[2]>=25000?s[2]:0)); Ps "Mach ports used by process \"$s\" with UID ${s[1]}"; o=`kextstat -kl | grep -v com\\.apple | cut -c53- | cut -d\< -f1`; Pm "Loaded extrinsic kernel extensions"; R && o=`sudo launchctl list | sed 1d | awk '!/0x|com\.(apple|openssh|vix\.cron)|org\.(amav|apac|calendarse|cups|dove|isc|ntp|post[fg]|x)/{print $3}'`; Pm "Extrinsic system jobs"; o=`launchctl list | sed 1d | awk '!/0x|com\.apple|org\.(x|openbsd)|\.[0-9]+$/{print $3}'`; Pm "Extrinsic agents"; o=`for d in {/,}L*/Lau*; do M; done | grep -v com\.apple\.CSConfig | while read f; do ID=$($PB\ :Label "$f") || ID="No job label"; printf "$Fb" "$f" "$ID"; done`; Pm "launchd items"; o=`for d in /{S*/,}L*/Star*; do M; done`; Pm "Startup items"; o=`find -L /S*/L*/E* {/,}L*/{A*d,Compon,Ex,In,Keyb,Mail,P*P,Qu,Scripti,Servi,Spo}* -type d -name Contents -prune | while read d; do ID=$($PB\ :CFBundleIdentifier "$d/Info.plist") || ID="No bundle ID"; egrep -qv "^com\.apple\.[^x]|Accusys|ArcMSR|ATTO|HDPro|HighPoint|driver\.stex|hp-fax|\.hpio|JMicron|microsoft\.MDI|print|SoftRAID" <<< $ID && printf "$Fb" "${d%/Contents}" "$ID"; done`; Pm "Extrinsic loadable bundles"; o=`find -L /u*/{,*/}lib -type f | while read f; do file -b "$f" | grep -qw shared && ! codesign -v "$f" && echo $f; done`; Pm "Unsigned shared libraries"; o=`launchctl getenv DYLD_INSERT_LIBRARIES`; Pm "Inserted libraries"; o=`find -L {,/u*/lo*}/e*/periodic -type f -mtime -10d`; Pm "Modified periodic scripts"; o=`scutil --proxy | grep Prox`; Pm "Proxies"; o=`scutil --dns | awk '/r\[0\] /{if ($NF !~ /^1(0|72\.(1[6-9]|2[0-9]|3[0-1])|92\.168)\./) print $NF; exit}'`; Ps "DNS"; R && o=`sudo profiles -P | grep : | wc -l`; Ps "Profiles"; for f in fstab sysctl.conf crontab launchd.conf; do Pc $f /etc/$f; done; Pc "hosts" <(grep -v 'host *$' /etc/hosts); Pc "User launchd" ~/.launchd*; R && Pc "Root crontab" <(sudo crontab -l); Pc "User crontab" <(crontab -l); R && o=`sudo defaults read com.apple.loginwindow LoginHook`; Pm "Login hook"; Pp "Global login items" /L*/P*/loginw* Path; Pp "User login items" L*/P*/*loginit* Name; Pp "Safari extensions" L*/Saf*/*/E*.plist Bundle | sed 's/\..*$//;s/-[1-9]$//'; o=`find ~ $TMPDIR.. \( -flags +sappnd,schg,uappnd,uchg -o ! -user $UID -o ! -perm -600 \) | wc -l`; Ps "Restricted user files"; cd; o=`SP Fonts | egrep "Valid: N|Duplicate: Y" | wc -l`; Ps "Font problems"; o=`find L*/{Con,Pref}* -type f ! -size 0 -name *.plist | while read f; do plutil -s "$f" >&- || echo $f; done`; Pm "Bad plists"; d=(Desktop L*/Keyc*); n=(20 7); for i in 0 1; do o=`find "${d[$i]}" -type f -maxdepth 1 | wc -l`; o=$((o<=n[$i]?0:o)); Ps "${d[$i]##*/} file count"; done; o=$((`date +%s`-t)); Ps "Elapsed time (s)"; } 2>/dev/null | pbcopy; exit 2>&-
Copy the selected text to the Clipboard by pressing the key combination command-C.
7. Launch the built-in Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
When you launch Terminal, a text window will open with a line already in it, ending either in a dollar sign ($) or a percent sign (%). If you get the percent sign, enter
exec sh
in the window and press return. You should then get a new line ending in a dollar sign.
Click anywhere in the Terminal window and paste (command-V). The text you pasted should vanish immediately. If it doesn't, press the return key.
If you're logged in as an administrator, you'll be prompted for your login password. Nothing will be displayed when you type it. You will not see the usual dots in place of typed characters. Make sure caps lock is off. Type carefully and then press return. You may get a one-time warning to be careful. If you make three failed attempts to enter the password, the test will run anyway, but it will produce less information. In most cases, the difference is not important. If you don't know your password, or if you prefer not to enter it, just press return three times at the password prompt.
If you're not logged in as an administrator, you won't be prompted for your password. The test will still run. It just won't do anything that takes administrator privileges.
The test may take up to a few minutes to run, depending on how many files you have and the speed of the computer. A computer that's abnormally slow may take longer to run the test. While it's running, there will be nothing in the Terminal window and no indication of progress. Wait for the line "[Process completed]" to appear. If you don't see it within half an hour or so, the test probably won't complete in a reasonable time. In that case, close the Terminal window and report your results. No harm will be done.
8. When the test is complete, quit Terminal. The results will have been copied to the Clipboard automatically. They are not shown in the Terminal window. Please don't copy anything from there. All you have to do is start a reply to this comment and then paste by pressing command-V again.
If any private information, such as your name or email address, appears in the results, anonymize it before posting. Usually that won't be necessary.
When you post the results, you might see the message, "You have included content in your post that is not permitted." It means that the forum software has misidentified something in the post as a violation of the rules. If it happens, please post the test results on Pastebin, then post a link here to the page you created.
Note: This is a public forum, and others may give you advice based on the results of the test. They speak only for themselves, and I don't necessarily agree with them.
Copyright © 2014 Linc Davis. As the sole author of this work, I reserve all rights to it except as provided in the Terms of Use of Apple Support Communities ("ASC"). Readers of ASC may copy it for their own personal use. Neither the whole nor any part may be redistributed.