No/Intermittent spark on #2 and #3 - follows signal wire from ignitor KZ500 LTD

Hi, trying to get my 1980 KZ550 LTD roadworthy. I can sometimes get a few minutes of very good running, but inevitably seem to end up with #2 and #3 not firing or fouling. Original wires, original coils. I've found that when I don't have any spark on #2/#3, I can swap the green and black "signal" wires going to the 2 coils and the problem follows the green signal wire.

I've traced the green wire back to the IC ignitor and it seems fine. I've taken the cover off the pickup sensors and stared at them (that didn't help).

Any ideas on what's the likely cause, or suggested troubleshooting steps?

One last note: I checked the battery voltage today at 4000 RPM, and it was only about 13.5 (new, charged battery). Doubt it's related, but it is below what I understand it should be.

Thanks in advance for any help

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