I have 3 2009 8 core mac pros (MB535LL/A), two of witch I have successfully upgraded the processor to two 6 core x5690. My last of the 3 mac pro was working fine, and then the dimm slots for processor A went out after several weeks. The computer boots fine and works well. I have torqued the heat sink screws perfectly, as the processors still work and have worked well in my other machines without any hick-ups. I have moved and reseated the ram and tried different configurations with no luck. I've even reseated the processors several times with no luck. I placed the bad processor trey into my other 2009 mac pro, and the dimm slots for processor A did not work. So it must be the processor board?? But here is the kicker, once the old processors (original Xeon E5520) are placed back into the machine the ram and dimms slots work perfectly. I have also seated multiple x5690 processors into the same board, and they all have processor A dimm slots fail. Any thoughts, suggestions, tips? Should I just get a new processor board? Buy new ram, although the ram works fine in my other machines. Thanks much!