Just to add my input into the discussion – you have checked a lot of the peripherals and I’m just trying to establish from the chat what you have done with the ECU – I would recommend that you get the ECU checked out by Roger Bywater at AJ6 Engineering – I don’t imaging that there is anyone else around who has half the knowledge of these early injection systems as he does, and I can’t really imagine anyone in any current repair shop understanding this (antique) system either. Check the AJ6 Engineering website for info.
I’m speaking from experience as well – my car was running fine (1986 3.6 manual cab), then started to develop erratic starting problems till it finally gave up the ghost. Roger at AJ6 discovered a lot of dry joints in the ECU (seems like the most common cause of any ECU problem) – upon refitting the repaired ECU, the engine was a different beast altogether – smoother, more responsive, and even more fuel efficient!
I think we try to imaging ECU problems as the last resort but if your car has the original ECU, then at over 30 yrs old, its likely to be suspect.
Just one other check first – does your car have the vacuum pipe connected to the ECU – make sure this is OK (not leaking) as well – the correct function of this has a big impact on the engine operation.
Hope this helps, all the best, Norman.