What other cards do you have in you machine at the moment - it sounds as if one of those is nabbing the IRQ before the graphics card.
Take a note of the IRQ the Graphics card is using (usually 11 or 9) and then have a look to see the value when the problem occurs. That way we should be able to identify the offending item.
Get a list of the IRQs by start -> Run -> WINMSD and having a look in the hardware, IRQs section.