Perhaps that's your issue, you must reduce memclock or either decrease the undervolt, increase power limit.
Bios is the last thing I would check (maybe your timings could be wrong too but just think about it when everything else has been tried).
I modified the opening post, adding the information about the cards: they're nvidias (my AMD rig runs fine).
As for the O/C, I'm currently a safe +140 core, +160 mem at 85 TDP