1994 vauxhall corsa/ holden barina misfiring

my Barney the Barina (that I have owned since new) decided to start misfiring if it rained… quick spray of wd40 on the distributor cap and he ran like the new again. Recently had some rainy weather, husband notices some grit in cap, sprays wd40, seems to be ok but then stalls/backfires/dies. Check distributor cap-burned out rotor, damage to cap. So far 1) replaced distributor cap, & 2)replaced rotor…still misfire. 3) replace spark plugs… still misfire 4)replace HT leads, still misfire. Barney starts, but is running rough. any Idea? @oldturkey03 ?