i took a known good stick and tried it in all four slots, and there was one that took a few tries, which isn't entirely unusual with old boards - they even just get dusty, sometimes. so, i'm going to try to leave it on with just the known good in the dusty slot and see if it halts or not.
these asus boards don't die (i have one from 1998 and one from 2006 that are both still running, and a third in a box from 2016), but they do get dusty and need to be reseated. let's hope it's that easy.
and, i'm going to have to start typing on the production machine, instead.