What differences were there in the Mark 2 and Mark 2a?

Very minor PSU tweaks, very minor audio input tweaks, and a change of RAM layout. It now has 2 x 64Mbit chips, rather than 6 x 16Mbit chips.

I'm getting the feeling that this problem, in the rare occasions it occurs, may be down to extremely low-level RF emitted by bus activity being picked up by insufficiently filtered receivers. The apparent fact that the 2A doesn't seem to cause it in a car where the 2 does might support this, as the smaller number of ram chips also requires less bus wiring length. This would have the effect of radiating somewhat less RF, since the effective antenna length would be completely different for bus signals. Also, if it's some sort of RF mixing process that's generating the offending frequency, different circuitry would be resonant at different frequencies. It seems feasible, anyway.

The serial cable is tucked behind my dash in a place I can get to it easially, and happens to sit right next to an electrical fuse box in the car.

That may be your main problem. The serial lead might act as a feeder, coupling any interference into the car wiring more inductively than any other way.

I have one suggestion that might be worth trying. Get a biggish ferrite toroid, about an inch and a half across, and wind a couple of turns of the serial lead through it near the back of the docking cage. Do the same thing with the main empeg wiring, ie disconnect the ISO connector, feed it through a toroid a couple of times, and reconnect it. See if this helps in any way. You should be able to get suitable toroids from Radio Shack or a similar electronics supplier.

Rob is right, though, the problem is most likely down to bad (cheap) RX design on the part of the car manufacturers.

Patrick.

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