I might have missed the fact if this is a Carb engine or fuel
injected, blowing smoke or not, pre-ignition, pinging.   Always do the
simple things first, swap $$$ECU$$$ last with a known good spare.  50
years with VWs, Porsche, Mercedes and BMW and no bad ECUs here but
lots of bad sensors.

Carb Engine Intermittent: With the engine running, I packed ICE on the
Hall amplifier then after an hour warmed it up with a hair dryer.
During the 60s, 70s and 80s this was a popular trouble shooting effort
for big computers.  Anyway, I found the problem quickly.  Sure the
problem could also  be fuel pressure (filter, pump, pinched line, air
hole, crud) or a bad ignition coil (cheeeep) or a broken or bad
resistor (cheep again) on the ignition coil for some cars.

Digi-fuel injection:  First, make sure all the fuel line components
are in good shape, good fuel pressure and no black smoke out of the
tail pipe cause that is a separate problem to do with the fuel
pressure regulator.   Make sure ALL electrical grounds are clean, all
electrical plugs clean and tight.  There is a very large ground post
under the dash board and under the drivers side engine .  Make sure
everything is clean and tight.

Engine Hiccups are well documented and has to do with the rheostat on
the side of the air box .  The moveable part inside develops a dead
spot in the resistor material and the engine literally hiccups ,
letting up or down on the gas peddle keeps the engine running.  I
drilled a tiny hole in the side of the rheostat and put a very short
shot of contact lubricant into the hole.  HOWEVER, read the
documentation, this problem is covered by many expert mechanics.

Bill89westy23