I doubt it is the AGP bus specifically that is causing your problem. I think it is more likely general instability, and the load from the game brings it to the surface.
In my opinion, your best bet for running the higher FSB speeds lies in better cooling for your northbridge. Have you pulled your northbridge HSF off to check and see if it has any thermal compound?
Improving the northbridge cooling allowed me to greatly increase the FSB on my old 8K3A+. Check my signature.
High system temps can cause strange symptoms as well, as the card, and even the MOSFETs and other mainboard components overheat. If it is in the 90's in here (Fahrenheit, happens often in the summer) and my case temp gets up to around 40C, I'll start to see strange flashes in games.