OK, I replaced the coil (about $17) and still had the problem. I then replaced the TFI module (about $35), and haven't seen the problem in a week! So I guess that was it.
It was kind of a pain to change, though. What should've been a 1-hour job took more like 3 hours. For starters, you can't fit a socket (11/16", by the way) straight down on the distributor clamp bolt. Besides, I didn't really want to take the distributor out, since then you have to deal with the mess of leaves/trash collected there that falls down into the hole. (I'd had it out once before.) So I tried for a while to just remove the module from the distributor while it was still in the car. It took me about 20 mins to realize that the reason I couldn't fit a socket into the recess for the bolts holding the TFI module on was because there was a plug covering the hole! Plus, my 7/32" socket wasn't a deep socket, so it didn't reach all the way down.
So, a quick trip to the hardware store to buy a set of 1/4" drive deep sockets (which I really should have had anyway). Only to find out that the socket I bought was about 0.002" too big in diameter to fit in the recess!! >:( grrrrr!!!!!! I don't have a grinder either (how on earth have I survived so long?), so I had to clamp it and file it down -- another 10-15 mins that I shouldn't have had to spend, modifying new tools!
Then, of course I find out that the 2nd bolt isn't accessible with it mounted on the engine, the way the distributor is oriented. So I have to take it out anyway. Fortunately, I found that with just the right extension on my socket (I do have SOME tools, after all!) I can get enough of a grip on the hold-down bolt to turn it off, even though the socket isn't straight-on. And sure enough, even though I've blown the daylights out of the area with compressed air, a bunch of grit manages to fall down. So another 10 minutes cleaning all of that out. It would've been nice to have some kind of oil wash system, to do a better job cleaning off the gear on the distributor.
Fortunately, I had marked the position of the rotor relative to the distributor housing, and I decided that, as long as that was maintained, it didn't matter at all how the assembly was oriented back into the engine block. Except that the TFI module wiring harness still had to reach. I was hoping to re-orient it so that the module could be removed without taking out the distributor, but that didn't seem to work out.
Anyway, it all went back together without too much cursing, and the car is running great again!