I would try the memtest, except this morning the computer failed to pass POST. There was a long beep followed by a short one, which the mobo manual says is a memory or motherboard error.
I tried clearing the CMOS again (remove mobo battery for > 1hr), but the error persists.
When I got the BSOD, the first thing I did was run memtest, it was about 2 days ago. I did not run it as you described, but it found no errors after 6 passes.
I have been using the 1066 settings.
I have removed ram modules and tried booting with various single sticks in slot 1, and had the same issue each time. I have 6 ram sticks, 2GB each. They were not all part of the same kit, they came in 3 sets of 2.
I have also discovered that I can reboot the computer with ctrl+alt+delete, even though nothing shows up on the screen. Could the system be passing POST but not showing a video output?