I wrote the entry below about 9:30 am. By 10am, I observed “ECC Correctable Errors: 48464” with module A2 (same module as before) immediately after starting a test run, and that was at an ambient temperature of 75° F—relatively cool. So it seems that the module in slot A2 is flaky.
SATech is going to exchange the modules for me, replacing them with ones using heat sinks meeting Apple’s specifications. I’ll follow up in a few days when I have the new modules.