I have a 1994 +8 and had a problem when the engine revs were declining e.g. coming to a halt it would cut out, is this similar to your issue? In my case it was the earth to the road speed sensor which had become loose and was giving intermittent connection. The code reader was showing code 48 which is the Stepper Motor or the Road Speed Sensor, the Stepper Motor sticking can cause rough running. Have a look at GOMOG: https://www.gomog.com/allmorgan/14CUXCodeReader.html

I have an 14CUX Fault Code Reader permanently attached to the ECU that came with the vehicle, I don't know where you could get one now.

Bruce
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice)
2013 M3W (Olga)