[PxTx]

Can you verify what your initial timing, total timing @ 3000 is? How much vacuum advance?

Any chance you could get it on an O-scope and examine the ignition system. You could also use a wide band O2 probe in the tailpipe and dial in the carb too.

The cylinder pressure at low speed is greater than in the upper R's and increased cylinder pressure = increased resistance in the ignition system. Would seem like it ignition related.