JP - this is exactly what happened to me as well!
I installed a second processor and got the same issue. And it was because of this that I replaced the motherboard.
While looking on the internet I found a few other references to the same case, so it seems the memory issues are related to the second-processor ones.
Either way - replacing the motherboard did fix all of those for me. When I get access to that machine I can check the mobo version - at least we'll know which version works.