Hi all, I know this might strictly be the right thread...but I'm hoping someone can at least point me in the right direction. 
I have a MacPro 4,1 which I have upgraded to 5,1 (I believe successfully, it shows 5,1 in the system profiler.)  I bought two x5680s on ebay, and tested them in a friends PC.  Both worked.  I then very carefully delidded them with a depth marked razor, and gentle use of heatgun.  No cut caps, no overheated die. 
However, when I put them in my CPU tray, I get no post.   No boing, nothing.  Machine turns on with power button, HDs spin up, DVD drive initializes...no error lights on CPU tray...but no boot.
I have swapped CPUs between slots, I have tried CPUs one at a time in A, I tried another x5680 that I have which has clearly cut caps, and it causes red error lights on the tray.  I put my original quad 2.26s back in, and it boots right up.
What should I try, or who/where should I ask of assistance?