lucky catch
So the problem I was working on when the system kept dropping me every few minutes last night was with how a part was set up to be edited. One version was set up the normal way, and one was set up for a creaky old update method called IEBUPDTE. The one using IEBUPDTE couldn't be saved correctly. I checked other parts in the same product and didn't find any others using IEBUPDTE, so the obvious fix was to change it to use the normal SPACMS setup (where SPA is the name of the application where I'm setting this up, and CMS UPDATE is the update method to use).
This morning I put together a quick query to look for other parts using the IEBUPDTE method. It was used in a bunch of other products, and in very old releases of this product, but once I eliminated those I came up with exactly one part. It was in the newest release, the only one I had set up, and guess what the part name was: SPACMS.
After blinking a few times, I realized what must have happened. I looked for part name IEBUPDTE and found it in every release but this one. So I had managed to change the part name instead of the edit key.
That's right, the creaky old update method we can't make SPA use correctly for this product anymore is part of this very product. Lucky that I had used a CHANGE command that only changed it once per line, lucky that I happened to get another problem related to this edit key, and lucky that this one wasn't buried in a list of hundreds of other parts set up to use it.