2008 Mac Pro overheating problem / screen freezing but can still use mouse

Hi there, 

So I've come across a problem that I can't seem to pinpoint on my 2008 Mac Pro/10.11.6/16GB. It initially started with the screen freezing, but the mouse would still move and audio would still be going. This would happen every day or so to start, then getting more frequent, and I'd have to manually restart with the power button each time. I've come across this problem in the past, and I’m guessing the GPU was overheating. What initially solved the problem for me was actually cleaning the dust off of the RAM/heatsinks in the compartment below the GPU, as it seemed those had gotten gunked up over the years, and were running super hot and the heat rose to the GPU right above. After I cleaned the dust out it worked for a couple years, but now the problem’s happening again. So I removed the dust off the GPU and RAM, and also the dust behind the CPU+RAM fan, as well as the dust in the superdrive compartment and the power supply. No dice. I've also reset PRAM and SMC, booted in safe mode and running disk utility on the HD, and run a hardware diagnostic test, to no avail. There’s 220GB free on my hard drive, so not low. 

In the process I discovered (via iStat and Macs Fan Control) that not only was the RAM and GPU getting hot, but so was the power supply. Right now the computer doing nothing intensive, and the RAM riser A slots are 80°C/74°C, and my PSU is 68°C/61°C. I’ve seen other people’s sensor readings saying that their power supply is 45° or 55° max. Is it possible that the power supply is sending out too much juice and causing other components to overheat? Or maybe my sensors are misreading? The graphics card and RAM are all really hot to the touch. Another curious thing that has me perplexed is that despite these hot readings, the computer’s fans are all running at the minimum. You’d think they’d be cranking up to cool down these hot temperatures, or is it something else which determines the fan speeds (like CPU load)? Anybody have any ideas? I know my computer is old, and pushing for Methuselah-status in computer years, but it still works fine for me and I’d love to figure out what’s going on and fix it.

Thanks.