Volvo Repair: 240 Cuts out when it rains, volvo 240, bell housing

Question
Volvo 240 1990
This car runs like a top when the humidity is low, but when it rains or the humidity is high, it cuts out. If we put STP gas treatment in it, the issue clears up.

We had a tune up recently and replaced the fuel filter. What should we do next?

Answer
Eric, Cap and Rotor usually clears that up as well as the Engine speed sensor that sits on top of the Trans bell housing.  If its chaffed or cracked, replace it.  It may also be ignition related.  Ignition Wires, etc.  What did they do? Put spark plugs in it?  Roger