Start up issue due to weather

i just had exact same problem. It got very wet and humid quickly after a cold snap. Mine's a 92 Custom Cruiser with 5.0 engine, 115Kmiles. Ran rough and wouldn't take throttle. Backfired a lot. Ran better on next day when it was dryer, but still not right. I removed distributor cap and found a LOT of white powder (corrosion) .. Near top was some black stuff looked like cobwebs between three of the plug wire taps and i believe that stuff absorbed moisture on our 100% humidity days. That let spark get to wrong cylinders , or just short to ground down through white powder. Have it almost back together. Everything was original, i'm replacing plugs, wires, cap and rotor so she'll go another twenty years. On firewall above distributor on passenger side is a little box that's the MAP sensor. It has a SMALL black tube that goes down behind throttle body. When that tube gets old and brittle and leaks(or the chipmunks eat it in two down behind aircleaner where you cant see) it'll give same engine roughness but no backfires. . That'll give 'check engine' light, moisture in distributor won't.. old jim

Yeah I've been trying to figure this thing out for a week now... it's nothing major at this point but when I had the heater off, the light was off... turned the heater on... the light came on.. so might be something with the heater now... hope not, I just replaced the heater core not to long ago. Personally I think its the TPS, that seems to be my best bet... everything else seems to be checking out fine.