This could realted to power distribution on the board as well. Turn off and unplug your computer, take each card (PCI, AGP, PCI Express, Etc.) and reseat them. This can clear the issue most of the time, if not you might have a conflict between hardware devices, repeat the process, removing the cards again, but this time go it one at a time with a "boot" in between to narrow down the issue. The most common I have seen is with the desktops that have a wireless card installed with an antenna in the reart of the machine, over time, friction and movment and jar the card out of the slot on the board and kill your video and other devices until it is put back in place.