G5 Roars Out of Control

I'm running a PowerMac G5 under OS 10.3.9. The unit works normally
most of the time, but I will occasionally find it first thing in the
morning roaring full blast, with the hard drive and/or fan just
spinning continuously at full speed and making quite a racket.
Stopping it requires a reboot. If anyone has any counsel on solving
this problem, I'd be very much obliged. Email reply preferred, many
thanks in advance.

Joe Vinikow
Seattle