For anyone who is still suffering from problems like what is mentioned above, especially if the picture freezes or turns black, or just messes up, and the sound keeps repeating itself or playing on, and there is no memory dump/BSOD, I've found that changing some graphics settings seems to help a lot, or even completely fix the problem. For one, playing Half-Life 2 and I think another game, this exact problem was happening, and I changed the in-game option to enable V-sync, and it seemed to fix the problem completely, so far. Also, for SLI, some people have suggested changing the option in the NVidia graphics settings that controls what kind of SLI is used in the game--split frame or alternate frame.
Also, my previous post in this thread is a little wrong--I still had some of the same problems after I underclocked my cards--they may have just been postponed.
I think this type of problem is likely caused by one of the following:
1. A configuration including a creative audio card along with SLI (or one VC), for which the problem may actually be the motherboard's fault
2. Overheating
3. Bad connections (they might come loose after a while) --unlikely
But who knows, really. All I know is that enabling V-sync has helped a lot, if not completely fixing the problem. I believe V-sync causes less graphics card work, and therefore less heat, but I think it also causes less bandwidth to be used on the motherboard, which seems to be fought over by Creative sound cards. The fighting over bandwidth might be what's causing our problems, as I've heard of other people having similar problems.