Your monitor should have a set resolution which it will work best at, any higher than it and your screen will become unviewable, and any lower and the screen will not properly fit the monitor.
You can try updating the drivers in the video card, and check to see that the monitor is connected properly and that none of the pins are bent or broken off.
You can also check the GPU temperature, but I doubt that's the issue.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w