Jackpot. I finally emptied out every table/index in all the DBCs, and reconstructed some test data from scratch today. Bingo, no problems (at least for now). This explains why it worked for a while previously, too.

Apparently NT is ultra-sensitive about something that Win9x is not. Something to do with indexes, I'm fairly certain. And I'm hoping NT is not *causing* the problem, but just manifesting it. I will continue testing to see if it breaks again.