Also possible that your monitor is picking up interferance from something. It can even happen if your monitor cable is running over/around power cords, etc. You don't see it often, but it can happen!
As cdogg suggests, try swapping monitors around and post the results back. There are other things that it can be also, and he has you on the right track. Mudskipper
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Groucho said it best- "A four year-old child could understand this! Quick! Run out and find me a four year-old child: I can't make heads nor tails out of this!"