I have a MacPro 3,1 and the screen keeps going black and the fan starts blowing at high speed.
I ruled out that it was my OS by running Clonezilla and attempting to clone the harddrive, and during the clone it eventually exhibited the same symptoms.
I was able to clone the hard drive on another machine with no issue.
I also plugged in, only a single stick of ram while trying to clone the hard drive and got the same results, so that leaves the logic board, the PSU and the CPUs.
I'm attempting to go the CPU route, but I can't get the heatsinks off to repaste them.
I've unscrewed all 4 screws on the heatsink for CPU A and I still can't lift it off the CPU, I hear alot of creaking down there, but it still won't lift off.