Colin Shepherd

The distributor cap is a likely candidate.
Also have a good look at the engine wiring harness insulation especially in areas where it gets very hot. Volvo fitted wiring looms with poor quality insulation for some years and the insulation tends to self destruct leaving bare wires inside the loom. This could cause problems after steam cleaning forced water into the loom.
Hope that is not the problem, but it might well be.
Colin.