I would do four things:
1) check the BIOS date on the compute and download and flash the BIOS if it isn’t up-to-date (http://www.asus.com.tw/Products/Motherboard/bios_slota.html). The latest driver fixes an “AGP mode control item.” You will be taking a chance if you flash the motherboard BIOS and you will be doing it at your own risk. If something goes wrong (e.g., a power outage) during this process you could end-up with an unbootable motherboard. If that happens you will have to find someone with the equipment to flash the BIOS, obtain another BIOS, or buy a new motherboard.
2) Make sure the VIA 4-in-1 drivers are installed, especially the AGP driver (http://www.viatech.com/jsp/en/dr/driver.jsp)
3) Download any driver updates for the AGP display adapter
4) Try another AGP display adapter