Sounds very much like a heat problem. The GeForce should have a very large fan on it. It should run when it hits a certain temperature. If it isn't, there's your problem. However, despite the fact that these newer graphic cards take a lot off of the processor they actually only do so at higher resolutions and most people play games at lower resolutions which means the processor still takes on most of the load. You have four hard disks plus ram that is highly tuned. That makes for a lot of heat not to mention that Athlon's just plain run hot. You may need a better heatsink/fan combo. Asus generally supplies a temperature measuring program. Monitor your CPU temps under load.

mm, i came to the same conclusion at first, was there with all my heat probing apps and prime 95 + a windowed copy of quake3
same result with unvarying temperatures

While I still think that this is caused by heat; you might look at the DMA support for your CD-RW drive. I've seen machines where the CD-RW needed DMA and only had PIO support knock a machine out for no reason even when they weren't be accessed.