Heatsinking question: Power Mac G5 PPC

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Heatsinking question: Power Mac G5 PPC

I got a PowerMac G5 dual 1.8 GHz PowerPC machine at a great price, so I thought I might play around with it a bit (I'm a PC guy). After acquiring the machine I read a large number of online complaints about these units failing due to 'thermal runaway.' It seems that Apple and IBM had some issues with these, to put it mildly.

I decided to investigate. I loaded a program called Temperature Monitor and observed the idling temps, about 41 degrees C on each CPU die. Then I loaded and ran a program called CPUTest to stress the CPUs and observe the temps. Both die temps, over about 4-5 minutes rose to about 58 degrees C. Next I pulled the CPU/heatsink modules out and examined them. The CPUs are mounted to small PC Boards with some support components, and strapped to the bottom of this huge passive heatsink. I pulled one assembly apart, cleaned off the existing thermal paste, carefully applied a 'silver' heatsink paste, reassembled the CPU and the machine.

Now when I test, both CPUs seem to idle at about the same temperature, 41 degrees C. When I start CPU test, however, the modified CPU jumps 10 degrees C to around 52 degrees C almost instantly, then rises to 58 or so and stays there. The other, unmodified CPU will start climbing slowly over 4-5 minutes to about the same temperature.

Apple wants their service dealers to run a thermal calibration program on the machine when CPUs are changed or problems arise, and I have done so. I know that the silver paste will take some time to 'settle in', up to around 200 hours accorcding to one online report I'd read. What I'm wondering is whether or not this fast temperature rise behavior is "normal." Any input appreciated...