GA-Z77-D3H system crashing/halting with video related errors.

[Tyrith]

Hi guys -- first post here, so I'll include a little bit of background, the system details and maybe someone can help me figure this out.

30+ years using computers, this is my .. 5th? or so PC that I put together myself and has been running great for the past 2-3 years.  I've done IT work in the past and am usually pretty good at figuring stuff out.  I'm still in the process of some of the tests but those'll be done over the next 2-3 days.  Right now my suspicions are towards the motherboard itself, and that's why the post.  Here goes:

Two video cards (Asus GTX 960, XFX 7870)
i7-3770k @ Stock speed
16gb Kingston 1866mhz ram @ Stock speed
OCZ SSD
Windows 7 64bit Ultimate
Gigabyte GA-Z77-D3H

The system randomly locks up on Youtube/Adobe flash related videos.  I find it tends to do this after running for several hours, although I have had issues of rebooting and immediately having the same problem.  First check was on the video card, I replaced the XFX 7870 with a 960, and within 4 hours of solid performance, had the same issue come back.  Both video cards have been through 3-4 different drivers with no fix.  Windows itself has also been reghosted, reformatted and reinstalled 3x.  I've seen stuff like this related to the temperature of the chipset on motherboards, and thought it was that.  The temperatures on the system are 40C/53C in the BIOS on reboot with the issue, so while I have not monitored heat on-the-fly, I suspect heat not to be the issue due to those being pretty low.

So, stuff that has been primarily ruled out:
Video cards
Operating system
Heat

Stuff that's going to happen over the next few days:
20+ hours prime95 on CPU
12+ hours on Memtest86

Assuming those come back with no issues -- what the heck else could it be?  I've never been in a scenario where I've been praying for a memory error lol.  The system was BSOD'ing on video card errors shortly after videos would show problems when I had the XFX 7870 installed, which is what prompted replacing it.  While I have not had a BSOD since, however, both Flash Player AND Windows Media Player were having major issues last night, which were fixed on a reboot.

Clueless at the moment, any help would be appreciated + will update in the next 48 hours on how the tests go.  If everything comes back positive on the tests the only things going untested would be something wonky on the motherboard that prime95 doesn't test (HDD would also be untested, but considering it took all the software reinstalls without something going wonky, I'm assuming it isn't that)  Also, any advice on testing utilities better than prime95 would be appreciated.  Thanks for any input.

[shadowsports]

Maybe this is my preference more than a necessity, but I would never SLI cards from two different manufacturers and expect long term stability.  That's another topic.

Tell us about the video related errors...  BSODs?, GPU recovery, errors in System Event Log...   what are they?

What happens if you go to one card?  Pick one.  That should be one of your tests.  Any recent changes, BIOS updates that precipitated the problem?  And tell us about your PSU, brand, wattage, age, etc.

[Tyrith]

What happens if you go to one card?  Pick one.  That should be one of your tests.  Any recent changes, BIOS updates that precipitated the problem?  And tell us about your PSU, brand, wattage, age, etc.

Both cards have been run in the system separately.  They are not being run at the same time.  They're being used as totally-different brands for the sole reason of ruling out the video card.

PSU is a Seasonic 1000W Gold+, about 2-3 years old, the same as every component in the system minus the new video card and ram.

As far as I know the BIOS is 6 months old and the problem showed up last week.  I'll post up the exact version shortly.  I'll post up some pics of what the videos are doing.  BSOD's are standard ATI-driver ones on the 7870, they don't BSOD on the 960GTX.  When it's having difficulty, it either "lags" up the youtube/flash video, goes "choppy" on Windows Media Player, or BSOD's when loading a game (hearthstone~) after the flash player goes goofy.  The game does not BSOD when using the 960.

Will try to have more info up later tonight after I go out prime95'ing.

[Tyrith]

Bios version: F16

Prime95 stable (small FFT's) for 16 hours as of this morning.  As far as my computer knowledge goes, that's the CPU/motherboard out of the picture.     What the heck else do I test if the RAM comes back fine?