We recently had problems with a new PC (Celeron P4 Motherboard) doing the following:
This was found to be due to incompatibility between some motherboards that are AGP 8X and 4X capable, and some AGP Video cards, that are 2X/4X but not 8X. The fault is in neither one, but between the two of them, they are a major problem.
The solution to this is:
Sorry there is no magic solution to this, but if you need to replace one of the other, your old parts will still be worth something 2nd hand.