Hi. I've got a problem for which I can't figure out the cause (there's nothing about it in Bentley or in the green manuals).

At idle, I've got the timing mark bouncing erratically back and forth between 0 and about 10 or 12 deg BTDC. But it steadies when I rev it above idle.

Should I blame the Distributor, or the Ignition Control Unit? Any recommendation?

Also, naturally, I'd like to swap in a good replacement -- do you have any from a parted car that you can sell me? Here are the details:

Engine: 1984 B23F, LH2.0 (automatic transmission), sold in New Jersey (meaning that it does NOT have EGR, if that's noteworthy).

Distributor: it's marked on the side, 237-508-003, and it has the larger 14mm diameter shaft (that one the rotor slips onto; and not to be confused with a variety with a smaller shaft, ~9mm). And it has to include the Hall sensor, one with a rectangular plug for the wiring harness (I think there's a variety with a round, cylindrical plug, but I wouldn't know how to convert it, if it was even possible).

Ignition Control Unit: all I can say is that it's marked 1317295 USA.

Thanks all, in advance, for your attention and help.