I had been having issues with my udm pro running hot and frequently overheating and powering off. It is out of warranty now and I wouldn't give ubiquity another dime but want to avoid replacing my network stack for as long as possible.
I was consistently running around 58-59c which I thought was hot but most things I read said it was fine. The problem was at increasingly more frequent intervals my network would go down and I would go to the data closet and would see "Ready to Power Off". Inspecting the logs after a reboot showed spikes to 102 before a shutdown was triggered. Most infuriating was that it wouldn't shut down but sit at the "Ready to Power Off" with fans at 100% and still hot.
I finally decided to replace the pads like I had read about other users doing. Upon inspection the existing pads seemed fine with one exception...
THERE WAS NO THERMAL PAD ON THE CPU!
Somehow they had put one on the 4 memory chips below the CPU, and one on top of the aluminum heatsink to presumably use the case as a thermal mass, but had somehow totally missed the damn processor! I have pics but they wouldn't upload. That means there was about a 2mm complete air gap between the heatsink and processor because I had to use a 3mm pad. I really don't know how it worked at all for the last few years.
Well gee, my temps are now in the mid 40s and it hasn't powered itself off since.
If anyone has an out of warranty UDM Pro that is running hot, you might want to open it up and check the pads. You will need 3mm for the processor and 4 mm for the chips and top of heatsink.