Sonofabitch, the problem appears to be the diagnostic tests themselves. I tried +/-10% combinations with both CPU and DRAM voltage and both sticks of RAM, with no different behavior.
Then I tried MemTest86+ 4.20
(released in January 2011), and it's running fine so far. Ubuntu 11.04 comes with MemTest86+ 4.10
(released April 2010). V4.20 "Added support for Intel Sandy Bridge CPU"
. I'm irritated that Ubuntu 11.04 and Win7 SP1 were released several months after the Sandy Bridge release, but neither has a memory test that's compatible. SB is a major architecture -it's not like they neglected the release of a Via or niche ARM processor.
Has anyone had succcess with V4.10 on a SB machine?
While I'm at it, I wish those two diagnostic tests would provide a more useful message
at the beginning. Something like, "CPU architecture is not recognized/supported. The tests may not function properly."
Would it be possible to build a RAM test directly into a new UEFI? That would be a great feature that I'd pay an extra $10 for.
Edit: 1 successful pass so far