IMac shuts down unexpectedly since Yosemite

Question

So, I apologize in advance for the long story:
I have a late-2009 27" iMac i7 with 8gb Ram and a 1TB HD. I updated my OS to Yosemite soon after it came out. Now, I admit I don't use the computer that often, so it took a while to notice the issues but, we first realized there was a problem in early November when my husband could not download a game (presumably because of the wi-if connectivity issues supposedly fixed in the latest update). It was around a week after he finally was able to download tthe game, that he was playing it and the computer would just shut down out of nowhere. No warning, just shut down and restart with the "your computer shut down unexpectedly" screen. Now, because it happened so many times, I started to get worried about all of my photos and music that is stored on thIs computer, so I went to do a back-up on my time capsule. This was when I noticed my computer had at some point been renamed (not by any human) to "my Computer (3)" when prior to yosemite (as far as I can tell) it was named "my computer". Also at this point I could not for the life of me get the time capsule to back up the computer as it was looking for "my computer" and all that was available was "my computer (3)". This is when it had been working flawlessly for years. I ultimately had to wipe the TC completely, reformat it and start a back up from scratch. It was for the short period of time that the time capsule was not connected to the computer that we noticed it was no longer shutting down unexpectedly. however, one the TC was reconnected and doing a back up, anything extra that you would be doing (with the exception of web surfing) caused the computer to crash. So, simple fix, right? Turn off the auto back ups while and try to remember to turn them on every so often. fine , dandy, whatever. However, I have done the latest yosemite update and the wi-if still does not seem to be staying connected, disconnecting every few minutes. Now I'm seemingly having more issues with a game that is well within the limits for my computer and the game will not start. There are just so many things going wrong with the computer that I'm really concerned that everything is going to disappear off of it one day. Anybody have any clues as to my computer's issue??

Answer 1

Try these in order testing your system after each to see if it's back to normal:
1. a. Resetting your Mac's PRAM and NVRAM
    b. Intel-based Macs: Resetting the System Management Controller (SMC)
2. Restart the computer in Safe Mode, then restart again, normally. If this doesn't help, then:
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the Utilities menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
3. Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the Utilities menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu. Select Restart from the Apple menu.
4. Reinstall Yosemite: Reboot from the Recovery HD. Select Reinstall OS X from the Utilities menu, and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible
            because it is three times faster than wireless.
5. Reinstall Yosemite from Scratch:
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
How to Clean Install OS X Yosemite
Note: You will need an active Internet connection. I suggest using Ethernet if possible
            because it is three times faster than wireless.