OK, it turns out this is harder than I expected. I'm fighting a very strange bug in my LOAD code. It's complaining of a checksum mismatch, but there doesn't actually seem to be one. And, weirder, my program loads anyway, despite the fact that the checksum failure branches away from the code that is supposed to reset the BASIC program and variable space.
In short, it's failing where it's supposed to succeed, and it's succeeding where it's supposed to fail. Wait, what?!
Whatever the bug is, I still intend to squash it before the Retrochallenge deadline on Thursday. Plenty of time! (he said, over-confidently)