If the return line test suggests the line is clear, I have some experience with moisture with M103 engine, but perhaps applicable here as well:
1. What is the condition of the ignition wires and their connectors? If poor, insulation cracking a bit, etc., just replace them. Start the car in the garage at night, switch off the light, just have some light at hand. Check if you can see blue sparks around ignittion system components. Replace what is necessary.
2. What is the condiditon of the ignition distriutor cap and rotor? If you look under the cap and you see greenish residue or other type of dirt inside next to connectors - you can clean it, but you may as well replace the cap if it looks old. I have once replaced my M103 cap with a Beru one. I fought with car stalling for a year. Then I bought a Bosch cap from Mercedes and the war was over, I won. I started buying caps from Mercedes only. Sometimes you may see thin cracks on distributor cap - thinner than a hair. if they are there, replace the cap.
3. Check/clean the points with a piece of fine sand paper.
