Missing when it rains

I have a 1999 Tahoe LT, 4Dr, 190,000 miles, however new engine - less than 20,000 miles on it. Runs fine except when it rains, has rained, or during snowy weather. In wet, or even high humidity conditions - it will run rough - seems to miss, or hesitates - just runs really rough. I realize this sounds like a distributor leaking, however it is not. I'm stumped, and so is my garage. It doesn't even have to be raining; if it has rained, it will begin to run rough - and will even run rough for about 30 minutes driving time the day after it was driven in wet weather. The problem has persisted now for several months, and I can't figure it out. I can put it in neutral, rev the engine and it will do the same thing. I have a cold-air intake on the vehicle also, however was installed long before this problem began. Although it has persisted for several months, it does appear to be getting worse lately - the rough running appears to be more severe than it was when it first started. Any ideas will be appreciated.

I have a 1999 Tahoe LT, 4Dr, 190,000 miles, however new engine - less than 20,000 miles on it. Runs fine except when it rains, has rained, or during snowy weather. In wet, or even high humidity conditions - it will run rough - seems to miss, or hesitates - just runs really rough. I realize this sounds like a distributor leaking, however it is not. I'm stumped, and so is my garage. It doesn't even have to be raining; if it has rained, it will begin to run rough - and will even run rough for about 30 minutes driving time the day after it was driven in wet weather. The problem has persisted now for several months, and I can't figure it out. I can put it in neutral, rev the engine and it will do the same thing. I have a cold-air intake on the vehicle also, however was installed long before this problem began. Although it has persisted for several months, it does appear to be getting worse lately - the rough running appears to be more severe than it was when it first started. Any ideas will be appreciated.

Plug wires ,dist, cap, coil or spark plugs.
Cracked or drawing moisture.

If it rains at night, start the car up and look under the hood for a arc, short, around ignition system. You should be able to see it. Or use a plant mister bottle. Duplicate the wetness. But MT-2500 is leading you down the right path.

so it runs bad when wet out . what did the replace or check ?

this is an old style 350 cu engine .