If you are using RivaTuner as well to overclock the cards or control fan speeds, you can get some very strange "side-effects". I found the same or similar issue with a dual-board GTX 295 folding alongside a single PCB GTX 295 under Windows 2003 x64. I would either get an "unstable_machine" on the second or third client, OR I'd just have it hang. I'd usually have to re-start the entire box.
IF you are using it, remove it and try again. If you are using EVGA Tuner the same problem exists because it is based on the same software as RivaTuner. The nVidia System tools didn't seem to have this issue, so you can try those if they work OK for you under Vista X64.
Best o' luck.
You might want to read the entire thread it mentions dual-pcb cards as well; Folding Forum • View topic - Pair of single PCB GTX295 video cards