the cap is drawing moisture from the rain. take the cap off and spray wd-40 in it and wipe it out. also change the rotor button under the cap too. it also might be the coil or the module inside the distributor.
ignighter [inside dist cap.] pop cap..sometime condensation gets inside,,,wipe out with wd 40,,,wipe down wires too,,,see if running close to rach other,,,could cross fire..