Try this:
Take out the AGP card and put it back in. Make sure that it is FIRMLY seated.
Do the same with your memory.
If this fails, do the same with the CPU.
Also, you should try this with only the vital copmponents installed on your computer. No network cards, no modems, no sound. Just AGP, HD, CPU, RAM. This would eliminate the possibilty with on of these cards causes some sort of hardware issue. (This happened with my SMC network card on my A7V).
Good luck.