My first thought would be that a dual core chip and a high end video card like that will be pumping out a lot of heat, so are you sure that you aren't suffering from heat related problems?
One quick way to test all this, is to remove the cover for a time and see if the games stabilize. If they do, you've got a heat problem that you should fix tout suite, because it will slowly but surely kill your system if you don't. Those are some rather high end parts you've got in there too, so I'm sure that system set you back quite a bit.
AMD's website should have information regarding normal operating temperatures for your CPU, and you can use the sensors on your motherboard to monitor this. You also want to keep the temperature inside the case as low as possible, since the air is what leeches most of the heat from all the different components, and one set of fans then expel the heated air out of the case, while another brings in fresh cool air.
If you have to, get a case with more fans. Some white noise from fans is preferable to several hundred dollars of kit being destroyed. Or you can look into water cooling rigs.
First of all my PC specs..
AMD x2 3800
PowerColor ATI Radeon X1800 XT 512MB
MSI K8N Neo4 Platinum nForce4 Ultra
2gb GeIL RAM
Hitachi Deskstar T7K250 NCQ 160GB
NEC ND4570 16x16 DVD