I would not think your new harness is bad, but it could be the sleeves that promote better contact at the computer plug and distributor connector may be missing. This was a change in mid-year 83 covered by a service bulletin. I expect if you've come this far you already are aware of it.

None of that really proves the hall-effect wiring inside the distributor is above reproach, though your wiggling seems to isolate the trouble to those things touched by the harness between computer and distributor. With the 83, the ECU will sense those sparks you created artificially with the jumper B-C and turn on the idle circuit and fuel pumps both, momentarily, but you need a sustained repeated, distributor-timed spark to keep the gas flowing.

I've never owned a white-cap, but read many accounts of success when the distributor was exchanged for a Bosch. Many Bosch were built with the round plug too, so the swap is plug and play. Post back if unfamiliar with the old SB on sleeves.