JP - that's an interesting idea but I doubt that this is the case. At the time when I originally tested that I had two Xeon processors - both x55 series - one was a 1.8GHz (I think quad-core) and the other was a 2.93GHz quad-core. At that time I repeated all tests with both processors - 3x8GB and 3x1GB with each of them and they both behaved the same - 3x1GB had no errors and 3x8GB reported errors. (I suppose it is possible that both of my processors were faulty.)
I suspect that more likely the case is some hardware issue related to the 5520 chipset, and which issue has subsequently been resolved in the newer versions of the board. (How do I tell which revision is my boar?) I suspect that there is also some limitation with the memory ranks - when I used the simplest single-rank memories all worked fine but when I moved up to dual-ranked ones it didn't work. I don't have any 2 and 4GB single-rank chips to test with but I suspect they'll work just fine.
Someone else mentioned earlier that they had a problem with remotely rebooting the machine and it stopping during boot with the error message - I resolved that by modifying the BIOS settings. (I forgot the exact name of the setting but it is in essence something like "stop on errors" which I set to No)
I ended up replacing the motherboard and that solved all of my issues.