So far it seems to be running stably after cleaning the SIMM sockets and chips with Deoxit. I'm surprised that reseating them didn't work, but the Deoxit did. None of the SIMM modules have gold plated pins, so maybe it did clean up the connection.

Combined total of 11.5 hours testing in memtest 86 without issues (13 passes total).

Will keep any eye on it as I continue working with the system, but it seems happy for now.