Recently overhauled my system with a new everything except for the case and optical drive. When it works, it runs like a dream, but it is constantly randomly crashing. There is no consistent reason for the crashing that I can tell - it can happen 5 minutes after boot, or after several hours of use, or after waking from sleep mode... It's happened watching videos on youtube, playing wow, opening word, or even just sitting idle on my desktop.
My components are:
Gigabyte GA-890GPA-UD3H rev 1
Phenom II X2 555 BE
Sapphire Radeon HD5770 Vapor-X 1gb
Caviar Black 500gb SataII
4gb (2x 2gb) Corsair XMS3 1600mhz
Corsair 650TX PSU
Windows 7 Ultimate x64
When the system freezes, any audio ends up looping, and I can't do anything other than a hard restart. When it's restarting, it's about a 50/50 chance that it will start properly - half the time it freezes on the windows logo (after all coloured dots come together) or it insists I go to startup repair, does absolutely nothing that I can tell, and then may or may not boot when it restarts again.
So far I've tried flashing my bios both to F6 and F7C (currently using F7C). I've updated the GPU drivers, with and without CCC. I've tried clean-installing Win7, I've done burn-in tests with and without my graphics card installed, run with and without Win7 power management, turned off all OC and tried with both "Fail-safe defaults" and "optimized defaults". I've uninstalled all but the 1 hard drive (I have an old IDE optical drive and an older Sata HDD), tried the drives in the Sata 3gb rather than the Sata 6gb ports.... I'm pretty much at my wits end.
This is only my 2nd desktop build, and I've pretty much exhausted my bag of tricks. Before I just conclude that my mobo is f'd and go get a new one, I thought I'd check here and see if anyone had some definitive advice.
 I tried to install Ubuntu x64 in the thought that this was a Windows 7 issue, but the system wouldn't boot from the disc (despite the fact that it booted fine in the other systems I tried it in). It installed fine from within windows, but I couldn't do a clean install.