Wild guess: Is it overheating? How hot is the ambient temp? Does it occur only during the day or at night when it is cooler too (or at least go longer at night)?
General diagnostics:
Try hooking it up to another machine and see if it still exhibits that behavior. If it does and you don't think it is overheating you have a bad monitor on your hands. Either way, possibly. I have never had a monitor overheat on me, but I live in Silicon Valley where the wheather is good. I have heard of it being a problem in hotter areas.