Howdy - Rather than start a new thread, I thought I would just add to this one and see what happens (plus it has a great title!). My G4 DP 1 GHZ is doing this too, but somewhat frequently. This is when it seems to happen - if I put the machine to sleep, sometimes it locks up (10.3.5). When I restart it from this it starts with the fan just blasting. Then I can put it back to sleep and wake it up and it is its normal "quiet" self. I disovered my 4 port USB card was loose and this seemed to correct the lock up when sleep issue for some time, but it happens now and then. My only recent change to this machine (prior to all this starting to happen) was installing iLife 4 - I mean, come on, that can't be related can it?
P.S. I do have the upgraded more quieter power supply for this machine from Apple, but just have not got around to installing it.