I have done a complete tune-up: plugs, wires, cap rotor, filters. The
oxygen sensor checks out to be ok from the resent post of the hot, cold
test.
What is it that causes the engine to have a slight miss and shake at
idle?

Gas mileage is between 18-21, and runs great warm, a little stumbling
when cold .

Since the bucking situation (thanks again all who helped) I installed
the in line diode from one of my other parts vans. I have not had
bucking since.

Thanks for any input

Rob

 1990 Gl 2.1, 120k