1. Remove cap
2. Get out tube of silicone and your silicone gun
3. Apply a bead of silicone on the edge of the cap
4. Re-install cap
That's what a lot of guys do here who loves wheeling in mud. Make sure you also block the cap vent hole with the silicone.
The downside is, over time water will condense inside the distributor, which causes corrotion and the exact same symptoms you're having now. But it isn't a huge pain to remove the cap every now and then and wipe it clean and check for corroded cap and rotor contact points.
1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...
1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***