What you have to remember is that you have got the original factory wiring looms/plugs and connectors which are all now at least 26 years old. Corrosion and dry joints etc can give electrical problems which are very hard to pinpoint as they are very often intermittent faults.
Before I had wasted spark ignition fitted on my 1990 2wd I replaced my engine, 044 fuel pump and radiator fans looms with updated modern ones and my car has not missed a beat since.