The new hall sensor didn't help.

I think the auto advance is not working correctly, the fuel mixture demands are changing as throttle is reduced, needing a richer and richer mixture from 4000 on down to idle. Attempts to adjust the mixture for idle result in a waaaay overly rich midrange, causing a cylinder to flood and drop out.

Spend more money time!

On a side note: I spoke to Adrian at CH today, he said that the Rcexl ignitions sold for the last 5 years or so have just as hot of spark as the CH ones. The only difference is a slight change in the timing curves between them. Even that, he said, is unnoticeable to all except the most discerning ear.