hi.

it's always the same card that are faulty ?
try switch cards on mb and check if it's the same card ... or the same slot-card who is faulty ?

I git this kibd of trouble, it was a weakness on my psu.
what is your PSU ? does the 12v is mono or multi-rail ?
if multi-rail, be sure that is not overloaded even if the total power of the PSU is not reached.