Like rkauer said, look for bad caps on the motherboard. They are fairly easy to replace and you can usually spot the ones that leak because the solder pads beneath are slightly oxidized. I have a finicky A1200 with an Apollo board that exhibits similar symptoms. Sometimes it runs fine, then gurus constantly. I shut it off, power on again and it may work for a while before guruing. I also noticed problems with the amount of fast RAM recognized; tried different simms but the same gurus kept happening. I'm probably going to eventually just replace all the caps on the board as they are all over 15 years old now.
