I have a 17in. G4 PB, this happened to one time. I had just viewed a file with the PowerPoint Viewer so classic started up, after I viewed the file and shut off the viewer, I thought classic shutdown. My fans came on and I looked in Activity monitor and some sevice related to classic was still running at 100%. I stopped the service and everything went back to normal including my temp. Anyway, keep the monitor on the dock, that way you can see when some service is using the cpu