This may sound crazy, but so be it. A friend of mine ran in to the EXACT same problem. When he went from 2 gigs of RAM to 4 gigs, it solved the problem.

He was trying to cheap out on the build. Started with 1 gig, it wouldn't run worth a damn. Went to 2 gigs and it ran 1 card fine, but the same issues as yourself when trying to run 2 cards.

I just confirmed all of this with him, so if your issues persist, add memory. And, as a side note, if it fixes the problem, please post a follow-up reply.

Oldgascan