Typical symptoms include random crashes in 3D mode, instantaneous crashes or hangs when entering 3D mode, image corruption, monitor blanking, computer freezes, and just about every other symptom that you might face with a video card.
Causes of the instability: There are two main sources of instability. First, there could be an incompatibility between the AGP card and the slot on the motherboard. Second, AGP instability will also be noticed on cards and motherboards con figured for 8x speeds. Sometimes, at this speed, the data transfer rate of 2133 MBps becomes too fast for the rest of the system to handle.
The motherboard is a complex knit of multiple components, talking to each other at varying speeds. When the CPU, RAM, PSU and other components are all perfectly matched and adjusted, AGP should work just fine. Any weakness in any of the links in this chain and problems are bound to show up. If you are facing reliability and stability problems with your system and you have narrowed it down to the AGP port, the solution is to verify card and motherboard compatibility and/or slow down AGP and see if that solves the problems. In rare cases, you might also face a memory resource conflict.