As you have tried another monitor we have to assume thats not the issue so it can be one of the following:
GFX card
The card could be faulty or have a faulty VGA/DVI connector. hard to prove unless oyu have spare cards or PCs around to let you swap bits around.
Cable
Easy to check. swap the VGA/DVI cable from PC to screen and see if its any different.
Settings
check the refresh rate as this often causes issues. Also worth checking what the recommended resolution of your monitor is as some resolutios cause them to go a bit mental.
Try changing the screen resolution and see if it still happens.