Is your video cable firmly attached at both ends? If it is, then it's
time to buy yourself a new video card.

To satisfy yourself that it is, in fact, the video card:
* Do you have the same problem with a different monitor? If the problem
doesn't manifest on a different monitor, you need a new monitor.
* Shut down your computer and detach the video cable (both ends).
Examine the pins - are they bent or broken? If they are, you need a new
video cable.
Otherwise you need a new video card.

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Leonard Grey
Errare humanum est