You might try removing the l.b. and refreshing the solder joints on the ram slots, which presumably go through the board to the other side. I don't know if that is the idea behind baking the thing or not — never heard that one before — but the whole heat and RAM business makes me think that cracked solder is the culprit. Maybe someone pushed too hard on a RAM chip.

The G5s look intimidating but they are really not that hard to take apart. You need a few tools, with which the processors come out and go back in just fine. There are some how-to guides out there describing the steps, though relating to servicing the liquid cooled units (the screws and so forth are in the same positions with the air cooled ones).

I had one of mine mostly apart in under half an hour — and fixed and working when it went back together. So it can be done.