engine management light & damp starting

306 1.6 petrol. For some years this car has had a tendency to display the engine management light when the dealer can find no fault. this has gradually developed into an apparently damp related problem in that if left out in damp weather the light displays and the engine does not run on all cylinders. if then left under cover a bit it seems to dry out and run normally.

my diagnostic skills stopped developing when my Hillman Imp went off the road and fuel injection and catalyst related ignition is a cold book.

I would have suspected an condensation/insulation related fault in the plug/coil leads when I used to do my own maintenance.

has anyone anyother constructive suggestions before i start replacing them? needless to say, having been totally gutless in a 2 mile run this morning, it is now running perfectly, although the light is still on.

Thanks