Sorry,I had it in my head you were running the Ford engine with VW controls but it's the other way around. I worked at a VW dealership in the engine/trans dept about 45 years ago. If the head to cylinder surfaces were not good we would lap them in just like doing valves. If that didn't do the trick there were annealed aluminum crush washers/gaskets that we would use. You are correct that there were no head gaskets as such from the factory.

Glad you found the ground issues and the TPS being glitchy. Fix that before you do more to it . You may find all is well then.

i would like to know how they are getting the stock tfi to function with all the bypassns you described. i wouldn't believe that to work but somehow they have.

auto enrich ( ae) is like the accel pump shot in a carb. its done a few diff ways but in your case ( with a stock ecu) its done via the vam and or tps. that vam is a very early and poor version of a mass air flow sensor, (maf sensor). they do work but there not as a instant response as tps/map based. my megasquirt ecu doesnt use a maf but rather just a map and tps for ae.

as fabr stated above.....replace the tps and verify it operating the correct voltage range. that should help the stumble. beyond that, would have to know what/why whomever bypassed the tfi. if they truly bypassed it then there must be something else controller timing. i wonder if they went with a mechanical advance from the older carb version??