IMac resetting

Question

Currently my iMac has a lot of trouble booting up. Every time it launches it seems to be installing an update (progress bar below the apple sign). It will get halfway and reset itself, or get halfway and go to grey screen then reset itself or just hang on the grey screen.
I can sometimes get it to go to recovery (still has progress bar but gets all the way to 100%). From there I've been able to reset and get in normally. But while operating it will randomly reset itself and the process begins again. I've been though the loop twice. On the first pass I repaired the HD and on the second I reinstalled Yosemite.
Resets have occurred while the computer has sat idle and while web-browsing. The second reset occurred while streaming some video and before resetting it just went to black screen and looped the last second of audio like a CD skipping.
The iMac was partitioned a while back for Bootcamp but has since been restored.
I have noticed the progress bar on boot up for some months but assumed this was a new OS feature.
Any ideas about what to check next would be most appreciated.

Answer 1

The progress bar during startup in Yosemite is normal. It replaces the old spinning gear.
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.