Unusual PPC PM 2.7 GHz G5 Startup Problem

Question

I downloaded the Service Manual for the G5 Macs, and in the troubleshooting section under the "no-startup" section, the only symptoms shown are ones where the chime is absent. My G5 always boots when cold (say 1 hour or more since last boot), and will run properly all day. There are no errors or error messages when the computer is running. But if run for a while (say 10 - 15 minutes or longer), it will not restart, or boot from a shutdown again, until it is off for an hour or more. The thing is, *the chime is always present* on any attempt at restart or re-boot. Then there is either a black screen, a grey screen with no Apple, or a grey screen with an Apple but no spinning gear. New 3.6V lithium battery, PMU reset, PRAM reset with no improvement.
To me this seems like a thermal problem... either a bad sensor or bad circuitry interfacing with a sensor. I have read of an "Ambient Board," a "Media Bay Sensor," and an "Air Deflector Sensor." These all seem to be very small PC boards with on-board thermal sensors plus circuitry. I removed and cleaned the "Ambient Board" because it was easy to access. The other two are really buried, so I haven't touched them.
I have "Hardware Monitor" by Marcel Bresink, and it reports all temperatures quite far below the "target" values after considerable running time. So it seems the problem is not an actual high temperature, but maybe a mis-read of one or more sensors.
I have eliminated software as a cause (I think) by re-formatting my HD and re-installing the original Mac OS X Tiger from the Apple Install Disc #1... same problem. The combo update to 10.4.11 didn't help, either. Running the "Apple Hardware Test" from Apple Install Disc #1 results in all "passes."
What does it mean if the chime is working? How about if the chime works and the Apple logo appears. but no spinning gear? Any comments?

Answer 1

Tried command-option-o-f after shutdown the last time I had been up & running. Got a black screen; one of the common failure modes. So apparently the Mac cannot get to the "Open Firmware" point when it fails to boot.
"Have you tried booting with the plastic door off? I had a similar problem - boot when cold, no boot when warm. With the door off, the computer would boot every time. After the login screen appeared I would put the doors back." - No, I have never tried that. Apple says the computer will not run properly with the plastic air deflector missing, although they might just mean it will not cool properly. The Service Manual mentions an "air deflector sensor label" that must be in place on the plastic air deflector. My computer has never had anyone else open it except me, but I will check for the presence of that label. Supposedly if the label is missing, the fans may run at high speed, but my Mac does not have that symptom.
What about the "Thermal Calibration" that the manual says is required if a processor is replaced or the processors are switched in their sockets? Never done on this Mac, but is it possible I need to have the Apple people do a thermal calibration? Would a drifted calibration cause this kind of problem? There is a disc image available to them which supports that calibration, but I cannot access it. I would need a "Service Provider" user ID and password to download it from Apple.