Hmm, kernel panics...
Have you updated anything recently software wise? A point update? New Apps?
Have you updated or changed anything hardware or peripheral wise? Added RAM? new USB device or something?
If so, and it ram fine before, these could veryy well be the issue...
If you have not, check out the kernal panic logs, and post them over in the PowerMac forum... I'm 100% sure someone will be able to trouble shoot it.
I would also run 'Disk Utility', fix permissions and also fsck... see if this helps.
Or you may just want to do an 'archive' install and see if that cures it... If not, I would say it was a hardware related issue.
Good luck,
Marc