Do you have another monitor to try? Also, do you have a friend's computer to plug your monitor in to see if it does the same thing?
I'm thinking your monitor is about to go out, but that is a guess with limited amount of information.
My system: (updated)
AMD 64 X2 4400+ (Fast!)
Asus A8N32 SLI Deluxe
2gigs OCZ DDR400/PC 3200 (Fast!)
WD Raptor 150gb SATA 10,000rpm
WD 250gig X 2 RAID 0
Thermaltake Tai-Chi case with W/C
BFG Tech Nvidia 7800GTX X 2 SLI\'d
<b>\"If everyone is thinking alike, then someone isn\'t thinking.\"</b> --General George S. Patton