No spark on 1 (of 2) coil at idle, runs fine above 3000rpm

I just found my source for a trouble idle that I have been having on my bike. I first thought it was engine related and posted the problem there. Here is a link to the thread with video of bike at idle:
Thread on engine forum
So, what is happening that is causing the bike to run fine over 3000rpm but not run on one of the coils at idle?

Thinking maybe old leaking plug wires and/or faulty plug wire caps might allow sufficient conductance at higher rpm but not enough for good spark at low rpm. Could test for leakage by spraying water mist over plug wires with engine running in a pitch black garage (watch for free fireworks show). Would also do a separate ohm reading on each plug cap. And if plug wires are long enough, clip back a bit to allow a fresh bite into their core when re-attaching the plug caps. Check primary and secondary coil windings. Should be approximately same for both coils. The secondary winding check is the high ohm reading between 1/4 plug caps on one coil and 2/3 plug caps on the other coil.

If existing plug wires are permanently fitted into the coils, and new Dyna coils are not financially feasible, new inexpensive Dyna solid metal core plug wires (delivered with caps already installed) may be fitted by using the plug wire splicers also available at Z1E (leave enough stub at the coil end to attach the splicer).

Meanwhile, if still have points style ignition, clean and gap points first, and then set timing using a strobe light (being certain the advancer unit is both retracting and advancing).

Good Hunting!