What monitor? How is it connected? Is the cable shielded? are you using an RTG card? Do you have any high powered electrical devices nearby? Is your screen sitting on top of a wall of magnets? etc....
Please forgive the slight sarcasm on that last question, the point I'm trying to make is that if you need help you need to provide as many details as you can. Giving us too much info is no crime. Not enough is no use.
Add some more info and your sure to get someone who can help, maybe me, maybe someone else, the more info, the better your chances.
Good luck.