Do you have access to a different monitor to eliminate that as a possible cause? What temp is your graphic card running at? If you do not know here is a free program to find out.
GPU-Z Video card GPU Information Utility
This will at least tell us if it is a heating issue on the GPU.
Also unhook your monitor cables and blow out the connectors then make sure it is seated firmly, you would be surprised at how many times things are related to dirty or loose cables.
Also is your monitor power supplied by the computer or a separate power brick?
How old is your computer? Laptop, desktop? Separate graphics card or integrated graphics card?