>As best I can tell, the serial connector and the tuner connector
>are not touching anything made of metal.

The results of the tuner loopback test STRONGLY suggest otherwise. As does the "after a cold (overnight) start it often works correctly" hypothesis.

Here's why it's going into AC mode: Hijack simply configures the Tuner serial port (on the tuner connector, not the regular serial port) and sends two bytes of data out on the TX line. It listens and receives two bytes from the RX line. It compares the two bytes, and if equal, it KNOWS that the tuner connector has been wired for loopback to signal "AC/Docked" operation.

Moisture from the air-conditioning, perhaps? Bare wires on the docking connector sometimes touching? The tuner connector brushing against something (it normally just dangles in space, so check it!) ?

I can easily "mask" the problem for you, but something may get permanently damaged if the short is never found/fixed.

Cheers