I have this problem many times most specially to my costumers, this is what i do(video card is the problem)
steps:
first of all check all hardware and connection to the motherboard if is running properly.
2nd UN-slot video card adapter clean the agp/pci slot and video card with paint brush take off all dust and cobweb on it,
3rd get a pencil eraser and the video card, rub the eraser to the gold part of video card do it side by side until its brighten/shine like gold.
4th slot again the video card to the mother board,check the other parts connection turn it on if all are ready,,
Good-luck!!
pm.. if what the result to you,,, thanks..