I may have solved my overheating problem in my G4 MDD. Taking a clue from Richard Mayer (see above) I added a new fan, almost identical to the old one in the G4. Initially it did not seem to make much difference. It still got too hot and ceased up, even after I tried several positions for the extra fan. Maybe the two fans were competing in some ways because it was difficult to position the 2nd fan so as to not compete partially with the old, original one.
Then one of my sons, Philip, who learnt basic computing on an old G3 of mine I gave him, and then went onto become a bit of a whizz on computers, Windows as well as Macs, suggested I try putting the new fan on the back of the G4 so it would suck instead of blow. This seems to be doing the trick. I positioned the new fan so as to be directly behind the fins of the heat sink. I have now had the G4 running for over 90 minutes with the temperatures of the CPU and the HD to reach a steady state of 48°C and 31°C respectively. The two fans are now working together, one blowing and the other sucking.
Main problem is the new fan is rather too noisy, but I guess I will have to put up with it if i want to continue using the G4.
Thanks to Richard for all the helpful suggestions.
Roy Clark
rclark@bigpond.net.au