I got the system to start up, but on the first boot it reported that the BIOS had been corrupted. The motherboard has a built-in recovery ability so after the BIOS was restored, the system posted fine and loaded ESXi.
The system ran for a little while but then crashed again. Purple screen but a new crash condition. (photo's attached)
Ran a memory test with MemTest86+, after 6 passes there were zero errors. Not an exhaustive test, but usually I've seen errors after one pass. I may try a memory swap with a stable system if I don't find the root problem elsewhere.
When I had the system running for a while I noticed some problems in vShere. On the Inventory panel most VM's are now listed as Unknown (see screenshot).
Also the Server 2003 that was running fine when I took that screenshot
now either blue screens while loading windows, or crashes at some point
after I login and get to the desktop. On one of the crashes I get an IRQL_NOT_LESS_OR_EQUAL blue screen...
I also have another VM that's unstable. I wonder if their instability is caused by the same issue, or if they've become corrupt because of the crash.