Without a doubt, you have a slow coolant leak. The cooling system sits right on top of the CPU’s and is slowly leaking. The good news: Apple will fix it for free.
My dual 2.7 from 2005 died in May 2009. I took it to an Apple store and was charged $85 for them to tell me that either the CPU or motherboard or both were faulty. I accepted this as an answer.
I put the computer away expecting one day to part it out on eBay. February 2010: I decide to part it out but first did a quick internet search on the machine’s value. I found a site that explained how to get these faulty machines fixed for free.
I took my machine apart (not easy) and found dried coolant between the CPU and cooling system. I called Apple and talked with their techies who ask me to send photos.
The case was verified and I was told to drop the machine back at the Apple Store. They put NEW CPU in a 5 year old computer! I’m still using the machine. Good Luck!
the website:
https://www.xlr8yourmac.com/systems/G5_coolant_leaks.html