Could be the video card going bad. I've seen bad cards do that.
Another thing is windows updates. Did you recently update your windows through the Windows Update feature? I've had windows updates totally trash my monitor drivers and had to reinstall the monitor.
1) Were you downloading/installing anything? Were you making any changes to the system settings?
2) Has any new hardware or software been added or installed recently?
3) Have you been getting any error messages? If so, then what do they say? It's a good rule to copy them or write them down.
Do you have another computer sitting around where you can switch monitors to see if the other monitor does it too. I'd hate to see you replace the video card and that not be the problem..sometimes it can be the drivers becoming corrupt. If you can switch to another monitor and see if does the same thing. Also, if possible, try your monitor on another computer...that will rule out if its the monitor itself.
Before proceding with replacing your video card or monitor, I suggest doing some research first and narrowing down what the problem could be.
