Those memory temps are about 15 degrees C higher than the ones I am accustomed to seeing reported under load. People are saying 74C normal load and 85C maximum when stress testing the RAM.
Something you are doing is pushing the machine harder than the heaviest uses I have come across. What are the drive temps?
If you have gone through drives and RAM on 4 machines, then the common factors are the usage and or the environment. It can't be simply a problem endemic to the MacPro model, because we're not seeing other people with this degree of problems, even customers who are doing 8 hour renders in Final Cut Pro.
Try, what I did, install the menu item smcFanControl 2.x (don't use the 1.x) and speed up your fans on the CPU/memory and exhaust and see if that gets rid of your ECC errors. I've been getting a few ECC errors when I run BOINC and SETI on my Mac Pro and I'm going to see if upping the fan speed gets rid of them. I tried AHT and it didn't report a problem.
I think Apple may have calibrated the fans slightly less than they need. But I'll see what happens.