Mac Pro 2009 2010 2012 dual CPU replacement phenomenon

[MacZoltan]

I got this phenomenon when i install a pair of Xeon X5690 without matching them sometimes the Mac Pro crashes or even does not boot at all.
Why is that so important and what revisions work together.
Stranger thing is these CPUs work together perfectly in server units like the Dell R710...
Any idea?

 

[Macschrauber]

Try to test them alone in Slot A and let B empty, fans go full.