After a couple of weeks of happy vertcoin mining with 5x EVGA 750ti's, I can't get anywhere now.  When Cudaminer first started crashing, I found it worked fine when I removed the two cards on powered risers. So then I mined with three cards for a few days. But now it just crashes on startup... black or orange screen, even a green screen once.  This happens with one, two, or three cards in the 16x slots.  I've switched them around, no luck.  I've reinstalled drivers, no luck. Stopped overclocking, no luck.

Win7 64 bit, Sempron processor.  Any suggestions?

Mobo is new, no apparent damage.  Power supply is new, only drawing about 60% of its capacity when everything is hashing.  I don't think either of these are the problem.

Helps?