Is your video card on your motherboard or is it a plug in, if it's on your motherboard (built in) it could be going bad. Same thing if it's a plug in, one thing (if it is a plug in (PCI, PCI-E or AGP) you might want to check out, open your case and look at the video card, make sure it's pushed all the way into the slot. I've seen this a number of times.
OR.....you could have a bad cable coming from your monitor, if that's the case it's cheeper to buy a new monitor than to get it fixed. Look at the pins on the end of your monitor cable, sometimes one can be bent or even broken off, more likely bent. If I could see the thing I could tell you better what the heck is wrong.