or there could be a problem with one of your RAM sticks
That was it apparently. I unseated all 4, tested each one individually. Got some weird results: 1 stick (corsair) froze the stress test in slot 1, the rest were tested in slot 1 just fine. Then I moved the bad stick to slot 4 and no crash... then the two corsairs together dual channel slots 3+4 and tested fine, then one of the ADATA's in slot 2 plus the corsairs, tested fine, and finally added the last stick to slot 1 and no crash in Prime95.
WTF? All I did in the long run was switch the pairs from 1+2 to 3+4 and vice versa but hey, it seems to have worked. Hopefully it stays that way.
Either way it was definitely a RAM error causing the lockups.