I'm having issues with games on my new computer. They sometimes work, and sometimes stall, and I think either DirectX or my onboard sound has something to do with it. Here's my specs:

Asus A8N-E (onboard Realtek AC'97 sound)
Asus EN7800 GTX
AMD A64 3200+
OCZ 2*512mb C2 memory
2*120gb SATA
1*200gb SATA

When I first built the machine, I installed Call Of Duty 2, and it stalled. I set DirectSound acceleration to Standard instead of Full, and then I could play. After a couple days however, it started stalling after loading a level, before rendering anything. I tried updating drivers, using different combinations, reinstalling Windows etc, but the problem remained. At times, the game would suddenly work, then without anything changing would stop working. This has also happened with Farcry, San Andreas, and Act of War. Quake 4 and F.E.A.R seem to run fine.

How should I go about investigating this problem? Any input is greatly appreciated, I would love to solve this without forking out a wad of cash.