so you've tried a known good monitor and known good video card? and are still receiving the same problem. it may sound strange but i've seen power supplies cause these problems before. see if you can't get your hands on a known good one to test. and of course double check all your connections to make sure they are tight. also, what type of video card are you using? ( PCI, PCIe, AGP) and have you tried a different type? or all the same in the same slot?