Time for a little more drastic measures me thinks. Like swapping some parts out or sending it to a pro. The fact it does well at higher RPM would generally make one believe the carb is getting and pumping plenty of fuel. Same on the ignition. Generally if they run at all, they'll start screwing up, for whatever reason, at higher rpm.
With no further clues, my best guesses would be some kind of vacuum leak, or a near complete failure/blockage in the carb's low speed circuit.
If unable to swap parts because this was my only gasser, I'd be tempted to contact Valley View to see if they might be willing to look at it.