Simply hook the monitor up to a different pc and see if it does it. If it does, then the monitor is going out. These things don't last forever unfortunately. Sadly, unless you're an electrical wizz, it's cheaper to just buy a new one most times than to have it worked on.
If you can't plug it into a different pc, then take it to a local computer parts retail shop and ask them to hook it up to one of their test machines.
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