1) Sometimes the inside of the monitor has a problem. Test to see if it is a monitor issue by following these steps: a)Turn your monitor OFF
b)Disconnect monitor from computer, leaving the power cord connected to the monitor.
c) Turn the monitor ON. You should see a thingy telling you to check the signal cable. If your monitor behaves as it has been, this may indicate a monitor problem. If not, reconnect everything.
2) Make absolutely certain you have compatible hardware.
3)Make sure your drivers are correct.