Hmmm....Is it hot in the room? that could make the vid card overheat......also, try a different driver if you can find one. Plus, you should make sure your AGP chip driver is up to date, check with your motherboard manufacturer. I know this is can be an issue with VIA chipsets and probably others...
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My System Specs:
Viewsonic E790B 19" Monitor
MSI 865PE Neo2 - FIS2R
P4 3.2Ghz HTT
Thermaltake Spark 7 HSF
2x512mb Kingston HyperX DDR400Mhz
Leadtek A350 TDH Ultra (FX 5900 Ultra)
(480Mhz core, 930Mhz memory)
35GB Maxtor/Quantum Ultra160 SCSI
Sound Blaster Audigy 2 ZS
Antec Performance Plus 660B case
400 Watt PSU
WinXP Pro