Here's a Macintouch page discussing "G5 Thermal Runaway". You might find something helpful in it.
On an advisory note, I had a 2.0 Ghz G5 Xserve for a while, which I understand uses basically the same CPU modules as the original G5 Desktops just with shorter heat sinks. I was warned at the time by the Apple engineer who gave the system to me that that the CPU modules were exceedingly delicate and that removing the heat sinks was a good way to destroy them. (Supposedly the naked CPU dies are *very susceptible* to being cracked or otherwise damaged by applying uneven pressure by over/under torquing the retention bolts.) For the record the Powermac G5 Service manual also claims that you *will* damage the processor if you separate it from the heat sink. Not "may", *will*.
If you Google you'll find people that *have* apparently gotten away with it (largely people trying to fix leaking 2.5Ghz watercooled models), but just be advised that from an official standpoint the magic smoke lives under the heat sink, no deeper. Of course, if it's broken anyway you don't have that much to lose.
The other thing to note that is in theory if you start swapping CPU modules you're *supposed* to run a "Thermal Calibration" program on them which is only officially available to licensed Apple servicemen. Undoubtedly there are... other ways... to find it, but that's an exercise for the reader. If you don't run that you may end up, among other things, with your CPU fans running at 100% speed *all the time*. (The Xserve I had would do that if it detected *any* anomaly in the cooling system, such as an unplugged fan on the other side of the case that had nothing to do with the CPU coolers.) Something to keep in mind if you start eBaying replacements.
In short, there are reasons the PowerMac G5 has a reputation of being something of a lemon. It is.
