I've recently upgraded loads of computer parts.
I have an Asrock 939 board, an amd x2 3800, and a bfg 7900 GTX, a SB live sound card, and 1gb of ram.

The card is obviously very good, but the problem is, during playing games, like Oblivion for instance, there is a lot of mouse lag, and mini freezes, not all the time, but it's there, it also crashes a lot, and when I turn it off after long periods of play, it crashes again, or just takes ages to return to my desktop. When I play doom 3, I get the same mouse lag, and some graphical smudging. It doesn't matter what resolution I play them on, I get the same problems, and even with maxed out graphics, I get great frames, but still have the mouse lag, jittering, freezing, and crashes.

I don't see a good reason why my system would do this, it's brand new, and decent enough to handle these games, so what could be going wrong?
I've ruled out sound being an issue, but I'm not an expert, I don't know what to check or how to remedy it.