try hooking monitor to the other PC.
This will help you to see if your monitor is misbehaving from some other hardware/OS in you PC.
If vice verso, try cleaning up all contacts on Motherboard with 100% alcohol and cotton sticks or use old tooth brush for AGP/PCI Express slots, and leave it to dry out for half hour or full hour.
Notice only to disconnect power cord before you perform this action!!!
This helper me once when I though my MB is dead