1/ checked to make sure the correct monitor is listed and not just Default or Plug n Play monitor.
2/You could be "over ranging" your refresh rate as well - so make sure your room mates haven't tried to tweak your graphics card.
3/ It could be a heating or power supply problem which is shutting down the graphics card when it is getting over worked. So check that fans are working. _ you could also remove any non essential hardware that uses power, eg PCI cards, to see if it is the power supply.
My monitor recently started just turning off. It's approximately 10 years old, so I just assumed it was dying. However, I have noticed that when I'm running certain programs, the monitor is just fine, other programs, it turns off every 30 seconds, and I have to push the power button twice, once to turn it off for real, once to turn it on again.
This inconsistency has me wondering if it may be a virus based problem, or a somehow damaged XP installation. It's a fairly recent install, and I have virus protection (AVG) - but on the other hand, I have roommates, and the computer has been open game, so I don't necessarily know what's happened to it.
Any ideas on how to clear up my monitor woes, or at least find out if the problem is the monitor, or the computer, would be much appreciated.