Well, I think I've inadvertently fixed the problem. It had also started to misfire at stop lights when the air conditioning was running. I was pretty sure the plugs and wires had never been changed, so I went ahead and swapped in some new ones. It not only fixed the misfire when stopped at a light, but it fixed the weird shifting, as well. I'm at kind of a loss to explain why it even fixed the problem. The only (admittedly non-technical) explanation I can come up with is that the misfiring plug was confusing the electronic controls. (One of the plug wire boots was bad and the electricity was traveling down the outside of the ceramic of the spark plug.) In any event, it's been several days now and it does appear that changing the plugs and wires fixed the problem.

Thanks for the responses, pcc!