I took my third machine apart, removed the cooler and pulled the CPU very carefully. I have a full SMT rework set of tools including a really good stereo microscope. I inspected the DIMM connectors and soldering on all slots and there are no bent or corroded pins anywhere. The soldering on the DIMMs were clean and solid.
I then inspected the LGA-1700 socket and every pin looked unbent and good zoomed in to max! IT takes a while to inspect every pin on an LGA-1700.
Then I looked at the bottom of the CPU. There was no thermal compound there at all and it looked clean at first. I could see small scratches on the pads where the socket pins made contact. But after closer inspection it appeared not to be as clean as I had hoped. I used some industrial grade cotton swabs and Deoxit Pro Gold (Has a different name now, and scrubbed the entire contact side of the cpu. There was some dust, but on some pads they looks a little bit dirty.
After cleaning very carefully, I reassembled the machine and now all the memory slots work perfectly. I think there must have been some bad contact between the pads and the socket contacts.
I would caution anyone to not try this at home unless you have the proper tools as damage to the fragile socket pins could easily occur.
I am quite thankful that I do not have to disassemble a second computer to check the CPU, worry about getting a replacement motherboard. What a pain that would have been.
Thanks for the suggestions. It helped me work through the troubleshooting sequence in my mind overnight.