New computer help

During springbreak, I bought a new computer and is great, for the most part. :?

its cpu is AMD athlon venice3200+(S393) on nforce 4 ultra with 6800GS from evga and 1gb of ocz ram. all programs run fine in the beginning, but after sometime running idle or just running some minor programs, the system chugs during 3d operations like games(even in less intense games like world of warcraft and galactic civilizations 2). The system idle is at 99, but cpu usage is at 100% also sometimes. When I reboot and run 3d games fresh, they run perfectly. but if i run the game after quitting the 3d program without rebooting, the system chugs. I checked for spywares and viruses and found none. I also updated my nforce 4 and 6800 drivers, but they didnt' help either. Can anyone help me please?

Also, I checked the fans for both the cpu and the vid card, and they are both spinning. Also, vid card temperature is at 34C. I don't think its cpu overheating because I played quake 4 4 hours straight right after boot just to make sure it wasn't the cooling problem.