Yes, I am using full 8 pin power. I agree, it sounds like not enough power - but then again it runs fine with the 280 in there.
@ Vaderat - Yes, I'll add them, but here they are again real quick:
QX9650 @ stock speeds and voltages, I think stock vcore is ~1.25v.
2 GB GSkill 6400 @ 2.0 - 5-5-5-15 (Tried 2 different sets of same RAM, also tried another Gskill DDr2 800 set, different model).
All other voltages set to Auto, except I tried setting NB (along with VTT) to 1.28 or 1.3, did not help anything.
Cooler Master 850W Real Power Pro
I did install the full Catalyst Suite, but this problem emerges before I ever get into Windows, since the computer cannot post at the same stable OC settings as I use when the GTX 280 is in the computer.
I have created a RMA label and I'm planning on getting a replacement board. This is just a very unusual situation that seems too much like hardware defect. Not even stable at stock? C'mon...
Thanks for your input though! :)