PowerEdge R640 stopped booting with second processor

Having received a large batch of RAM, I used an R640 which was not in production, fully loaded with 12 sticks at a time,  to test the RAM using Dell Diags - reloaded again and again - no problem, then on one of the RAM re-populations, it would not boot.

Only way to obtain regular boot was to remove second processor - then otherwise boots OK.

Error in idrac log "the system board pfault fail safe voltage"

Worse - this has now happened to a second R640 - exact same way.   

By the way all the RAM did pass tests as that looked like the obvious culprit.

I have tried other PSUs and even CPUs, but only the RAM was being exchanged on the task, nothing else was being disturbed.  Also have since cleared NVRAM and removed CR2032, only OK on one CPU, but not second.

With cover off and 2x CPUs in place, I note on "power on" - the chassis fans spin for about 10 seconds and lights on MOBO  near PERC - back ones stay on and front ones start flashing.

Any ideas on cause of this and how to resolve?