What istat version are you using? I had similar issues with istat using versions previous to 3 - bizarre drive bay temp readings (+1000F) as well as poor computer performance over all due to a memory leak in previous builds. Using istat v3 I now have accurate behavior & much better performance on my G5.
I dont know which computer model you have but perhaps one of the sensors has failed? This happened to my HDD temp sensor on my 2006 intel imac - was a super easy fix, but may be something you could check. As far as CPUs, it might be time to refresh the thermal paste on these older machines, so you could check the hdd/ambient/cpu sensors (if it has them), the CPU fan and redo your paste at the same time.
Good luck.