I actually had the AGP slot on my old mobo go bad on me. I traced the problem to having bad capacitors on the mobo. The capacitors that went bad first were the ones that supplied power to the agp slot. I was lucky that I had an old PCI video card that i was able to use until I got a replacement mobo.
In case you aren't aware of bad capacitors.
http://www.badcaps.net/