There are any number of reasons why a video card could play up but the favourite is always a loose connection. And just because it worded first, does mean it will continue to work. Assuming that you now have a PCI card, you will, of course, have to adjust your BIOS settings to restart the AGP slot, then put your card in and reboot. If that fails, double check that its fully seated in the slot - this can be very hard with a new mobo, but many cards have a locking clip - make sure that's fully home. If that fails, then its time to take it back to the retailer for checking!!