[MPDano]

Are you sure you got all the air/vac leaks sorted? An 84 needs all new rubber. A full intake refresh is in order for this car.

Also, not sure in the UK, but you can get those Injectors rebuilt and flow matched/tested to verify they are fine. I do know I had a Mitsubishi Eclipse Turbo that the injectors flowed perfectly, but once it got to temp, it started to miss really bad. And this was after all the injectors were tested.

Does this model have a Green Wire? If so, what color is it now?

Post pics of the car and especially the eingine compartment.