Funny motherboard DIMM issues

Hi there,

I am trying to add memory to my beloved T-7910.   I have noticed that some DIMMs work on certain slots and not others, and that some DIMMs work for CPU 0, but only if there are other DIMMs in CPU1.   I use BIOS A34.
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The errors are either code 27, or, more likely, system turning on but not booting: not even displaying the Dell logo. 

Any ideas would be greatly appreciated!