In most cases you won't see problems with CPU temps in the high 70's
IME the only real problem with that is that it'll decrease the lifespan of the CPU
In most cases this'll be a problem with the cooling solution:
- ensure that there's enough thermal compound between the CPU and the cooler (but not too much!)
- ensure that the cooler is free of dirt, dust, hairballs, etc
- ensure that the cooler fan is functioning and is blowing air into the cooler
- ensure that the air going into the cooler isn't heated by another device before it goes into the cooler
If you have further issues, it'd be better to get the hardware pro's involved by posting here: http://www.bleepingcomputer.com/forums/f/7/internal-hardware/
But, when running Prime95, you can safely let it get into the mid-90's before starting to panic. Most CPU's are safe into the 100's - but I discourage stressing them that much (and have found, in isolated instances, problems at 95 or higher).
The point here is that you're looking for the temps to stabilize while running Prime95 - if they stabilize in the 70's or lower, then the cooling solution is generally OK as it is. If it's higher, but less than the mid-90's, then it probably needs some attention - but it should be OK after that.
Which HDD test couldn't you run?
This still seems to be video related, and as you've tried different video cards, it's most likely related to how the video is processed on the motherboard.
As such, the definitive test would be to try another motherboard.
Lot's of CD-ROM controller errors - this is usually the same controller that runs the HDD
But it's not giving errors often enough to make it a suspect (only saw 2 instances, both on the evening of the 14th - sometimes this can be due to a bad CD/DVD)
Your UEFI/BIOS (version F8) dates from 2010. Please check at the manufacturer's website to see if there are any UEFI/BIOS updates available for your system. If you are able to install the update through Windows (without booting from an external drive), then go ahead and update it. WARNING - if the computer might shut down during this procedure, please don't do it, as this may physically damage the computer and prevent it from booting.
With older BIOS's, I have to wonder about compatibility with W8/8.1/10 - as it communicates more with the BIOS than previous OS's
Only 4 Windows Update hotfixes installed. Most build 10586 (TH2/1511) systems have more than this. Please visit Windows Update and get ALL available Windows Updates.
The actual number is not important. Rather it's important that you checked manually, installed any available updates, and didn't experience any errors when checking or updating.
On 15 April you did have a video driver failure (STOP 0x141: VIDEO_ENGINE_TIMEOUT_DETECTED )
I'd suggest following these troubleshooting suggestions: http://www.sysnative.com/forums/bsod-kernel-dump-analysis-debugging-information/35-video-tdr-timeout-0x116-0x117.html#post29532
Other than that, the system appears fairly "clean" of 3rd party stuff - so it's not likely that it's suspect (other than the things that I mentioned already).
As such, if you fix all the stuff that I suggested and you still have problems, then it's likely the motherboard.
It's your call if you wish to purchase one to test with - but be sure to get it from a place that will let you return it if it's not needed (and will give you your money back).
Remember that it's got to be compatible with your other hardware (memory/RAM, CPU, video card, etc) - so be careful when choosing.