Other threads suggest a faulty ABS sensor, connector, or ground. It’s apparently an electrical issue that the car interprets as a traction control and/or gearbox failure, although it’s neither. For now, I’m just disconnecting and reconnecting the battery to erase the fault when it occurs until I can get the car to my mechanic.

 

PSA: The apparent cause of my electrical issues was a battery cut-off switch that I installed on the main ground in the trunk. Since the issue was intermittent, it's hard to confirm this was the actual problem but, so far, the car has operated without issue for a couple of weeks. The switch I installed was the one that's been around for ever: green plastic round screw knob on top of two "brass" plates. Loosen the knob to disconnect, tighten the knob to connect. Apparently the mating surface under the knob got some slight corrosion or debris and made a poor connection.