I'd just like to say that I have the exact same problem as incubus. I returned my Gigabyte MA770T-UD3P a few weeks ago because it had some network difficulties, and finally got my substitute today, a Gigabyte GA-990FXA-UD3. On my last board, my Phenom II X2 550 could easily be unlocked to a quad-core and overclocked to 3.7 GHz and still be completely stable in Prime95 for 12 hours. Now, those settings cause it to crash the very instant I try to do pretty much anything, and it doesn't fare much better as a triple-core. This is seriously annoying. I didn't return my old board to get drastically reduced performance. Is there any way I can make my CPU work on my new board as an overclocked quad-core?
I have the same cpu and the ga-990 fxa ud3 rev 3 and i unlock my cpu to 3 cores with llc=extreme and vcore=0.100 mv and it's prime stable for 1-2h. When I try all 4 cores it crashes. The problem is with my video card or with directx and 3 cores the pc crashes. The pc crashes also with 3 cores and vmware. I already contact gbt to no avail. It's also anyoing that my x.m.p ram and the latest bios for rev 3 doesn't support it and it runs only at 600 mhz.