Do you see rapid changes in the RPM values of the fans, sometimes "cycling"
around the expected value?
If yes, this is a known bug in the platform drivers for systems of the
PowerMac7,2 and PowerMac7,3 series. The internal control loop implemented
by Apple is setup too sensitively and begins to oscillate.
Operating system versions prior to 10.3.8 and later than 10.4.3 are affected
by this problem. It's made Apple aware of this in October 2005, but obviously
they didn't fix it.
A workaround is to use an operating system between Mac OS X 10.3.8 and 10.4.2.