The only cheat I have active is the subscreen fix. The save state's been working fine the whole time. And I've played this ROM before, all the way through, and never got an error. Of course, that was before my computer installed Service Pack 2, which "upgraded" my graphics controller.
EDIT: I've tried the standard N64 save, and saved it over my Save State. But I think from now on, I'm going to try saving games with saves from the N64, instead of save states.