One of the machines has 5 GB and the other 8 GB. As part of a burn-in process, I ran two setiathome jobs on each machine. One machine was fine, but the other (8 GB) machine would hang after a while (5 - 30 minutes) under load. Worse yet, one or more sets of fans would go into "turbo mode" (very loud) and would need to be unplugged to stop. No system / console log entries were made regarding the problem.
All sticks of RAM were registered properly in System Profiler, and when idling or under light load there was no problem.
I had two brands of RAM (Edge and OWC) and initially thought that the combination might be a problem (it was not). I then went through an extensive troubleshooting effort and isolated the bad RAM to a single pair. The funny thing was that once I got down to a single bad pair the machine would finally panic before it hung. It did not panic when there were multiple pair of RAM.
Read the rest of the hint for more on the troubleshooting and repair process...
The AppleCare people seemed unaware of this type of problem. The seemed especially disinterested when I mentioned the "setiathome stress test" (although we routinely use this to burn-in equipment and it often finds problems).
The other thing that I found caused the fans to go into turbo mode was if the processor fans were not completely seated (this happened once on the 5 GB machine). The plate that the fans slide into is somewhat flimsy. The machine started fine but there was a red light visible at the front of the machine (near the power switch) and the fans were again very loud. And again, no system / console log entries were made and the machine ran fine for the few minutes that I was troubleshooting it. After reseating the fan it was fine.
Hopefully this information will save someone time (or a trip to the Genius Bar).
If anyone cares, the G5 processes a setiathome work unit in roughly 60-65 minutes per work unit (per processor, I run 2 jobs at once). My P4 1.9 GHz machine averages around 1.5 hours. Not bad.

