Could be a problem with the map sensor but best to check all sensor earths and make sure they are all fine.
Pinching the map sensor line - the result seems odd as it should be the opposite to what is happening. Cutting vac to the map sensor will cause it to run in limp mode and give a default fueling map, and it will miss under constant throttle.
Could well be a faulty map sensor, or the ecu could be old and capacitors leaking. I would take the cover off the ecu, inspect for bulging capacitors and if possible swap ecu's with someone.