Intermittent surging. (Revs go high during gear change, vehicle occasionally "Keeps going" when foot is taken off)

"Fluttery" power delivery at light throttle. (Hold back sometimes, but you can drive through it by putting the foot down)

At idle, occasionally drops intermittently to 3 cylinders. but revs freely and there is no evidence of misfire under load,

No smoke in exhaust.

Initially there were faults listed for Glow plug (confirmed) Throttle pedal and brake switch.

Fuel filter was also full of green slime and air was getting into the fuel down stream of the filter.

I have changed the fuel filter and air bubbles are no longer present.

I also changed the brake switch on the grounds that it was only a tenner.

The only recurrent fault code I have now is the known glow plug one.

However the surging/fairing, Fluttery behaviour and the missing cylinder at idle are still present.

My instinct is going for EGR and/or an injector but we are at the point where even looking to check is going to involve a hell of a lot of work.

Has anybody seen this before and can offer any suggestions as to where I should go next in order to minimise the possibility of futile work?