Hello,
Is the CPU overclocked?
Have you installed the latest BIOS and chipset device drivers for the motherboard, and the latest device drivers for the video card?
Are you certain the 500-Watt power supply is sufficient to power all of the components in your computer including the new nVidia 7800GS AGP video card? Also, could the CPU be overheating? If so, you may need to replace its heat sink/fan and possibly replace or install additional fans inside the computer chassis to increase the amount of air which flows through it to move waste heat away from the components inside.
Regards,
Aryeh Goretsky
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