What is causing a PowerMac G5/2.3GHz Dual Processor sudden abrupt shutdowns

Question

Hi all
I am posting in hope that someone out there can help me solve this very puzzling problem.
The machine: A PowerMac dual processor G5/2.3GHz (early 2005). 1.5G RAM. 250G HD. SuperDrive. Radeon 9650.
The problem: When under some load, this machine will suddenly and abruptly shutdown as though someone pulled the power cord. There is no warning. Just a soft click, and it powers off. The machine can be manually restarted after the sudden shutdown with no problems.
This problem is very replicable. (eg go to a particular website, play an embedded video, and boom, the machine shutsdown. Each and every time.) It was replicated at an Apple Store and at two Apple Authorized Service Providers.
System.log said "PMU forced shutdown, cause = -122"
Before the sudden shutdown, the machine seemed to be operating normally. The fans did not noticeably speed up. Activity Monitor showed no more than 50% CPU activity for each CPU. Temperature on both CPU was around at 70C before the sudden shutdown. Temperature and activity on both CPU goes up and down in tendem.
If I use the Energy Saver system preference to change "Processor performance" to "Reduced", the problem goes away. (As measured by going to the same website and playing the same video. Even playing 5 videos simultaneously did not cause a sudden shutdown.)
What have I done #1: I have reformat the hard drive with a 7-pass security wipe, followed by a clean-install of Tiger which was then brought up-to-date using Software Update. I then installed Safari 4.0.3. No other software is on the machine except Tiger and Safari. (Later installed Hardware Monitor to get me a handle on the temperatures.) The problem persisted.
What have I done #2: I then installed a new PRAM battery and reset PMU using the switch on the motherboard. Resetting the PMU also resetted the PRAM. The problem persisted.
What have I done #3: I then took the machine to an Apple Store. They found that they can replicate the problem easily. But, after 3 days of running the Apple Service Diagnostic, the diagnostic software didn't find any problem. They also did a thermal re-calibration. That didn't help either. Without an error code from the diagnostic software, the only "solution" the Apple Genius offered was to start swapping CPU, power supply and mother board at my expense. And even then, he has no idea which component to start swapping first. (Two independent Apple Authorized Service Providers offered the same non-diagnosis and roughly the same "solution".)
Has anyone encountered such a problem before? What was the solution?
How about an educated guess as to what's wrong?
I want to, if possible, and if reasonable, repair this machine.
Thanks in advance.
Lara

Answer 1

This sounds almost exactly like the problem I have. My G5 is also an Early 05 2.3G. I tried every tip I found on the forums from UPS vs. wall socket to stock RAM vs. 3rd party to booting from external HDs, etc.. I currently have 10.5.8 on it and it's running using the "Reduced" processor setting in Energy Saver prefs.
I have also made a trip to The Apple Store where the AS folks ran their diagnostics but were usable to replicate my problem. I understand they "can't fix it if it ain't broke".
Aside from the "Reduced" setting work-around has ANYONE actually solved this problem definitively? Although I have no interest in spending $1000 to fix this, I can't imagine disposing of it on Ebay.
Assuming this is a hardware issue, this would mark the first insurmountable failure of any of the 10 or so Macs I have owned.
Anyone? Anyone?