After reading and pondering for 'lo these many weeks, I am of the Opinion (notice the capital "O") that your problems are distributor oriented.

I say this as you have noted previously that your SU's were rebuilt and tested by someone who we all trusted to do things correctly.

I am obliged to ask "what model" distributor you have chosen to bolt into place. This question is based upon reference to vacuum - which the stock PV544 equipped with the Bosch 003 or 009 mechanical advance, had no need for.

If you have "upgraded" to a vacuum advance (or worse, retard) distributor you may have problems associated with the new and improved distributor working correctly - especially as the vacuum must come from a specific nipple on the rear HS6 carb body as fitted to the later model B18/20 engines equipped with the later model distributors.

A couple other options for error. 1) Distributor shaft inserted 180 degrees out. 2) Distributor shaft lobe worn. 3) Timing gear (thanks to the cut of the gears) installed one tooth off. 4) Plug wires/Coil wire shorting out.

Maybe I am odd for defending the venerable SU HS6, but in five years I have not had one bit of trouble with my set.