OS 9 (not Classic) crashes iMac

I've had this problem twice now. I choose OS 9 in "Startup Disk". I restart. I use OS 9. Something happens that forces me to restart. I do, and the iMac can't find either OS X or OS 9. I get a folder icon with a question mark. I then have to run Tech Tool to repair the disk before I can see either disk.
I rarely use OS 9, so it's not really an issue, but every once in a while I need to boot up in OS 9. I've tried re-installing OS 9, and OS X, but to no avail. Is this just an issue of the iMac being incompatible? Anyone ese had similar issues?

Perhaps you dont have enuf power going to your mac? I had a similar problem with my G4.. had so many things pluged into 1 outlet that it would crash at random and restart funny

That is an interesting theory. I do have a bunch of stuff plugged in: three firewire devices, line out to the stereo, two USB devices.

try putting those on a different plug or outlet if you can.