I am not sure if this will help others but I might as well put down my story of a PowerMac G5 desktop machine that suddenly showed complete freeze-outs more or less soon after booting here:
The G5 ran for years flawlessly and was brought up to MacOS 10.5. along its timeline. We unsuccessfully tried all sorts of hard- and software tests and utilities to localize the problem. Reading reports about bad soldering joints underneath the processor we sent the machine to a company that is capable to solve such problems by resoldering motherboards. Strangely enough the company wasn't able to reproduce the problem. It seemed to have vanished by the shipping process. After the G5 was shipped back to us the same old problems occurred again. Furthermore the little fan on the ATI Radeon graphic card processor made a lot of noise too. After closer inspection it turned out that this little processor fan did not always start spinning upon starting up the machine. Because an in-house IT-specialist was able to locate the same type of proprietary fan on eBay as a replacement we were able to fix the problem by exchanging the fan. The machine runs like a charm since the replacement again as it did for many years before!
Moral of the story: Check all fans if they spin correctly after booting a G5 Apple that randomly stalls.
We learned the lesson the hard way.. it's all about temperature!
Urs the Apple LabVIEWer