Random multiple cylinder misfire when wet/damp outside

[straight6chevyguy]

Hi guys my truck has been having some engine misfire problems lately.
Truck is a 1998 Chevy 1500 4.3 V6 5 speed manual.

When ever the weather is wet or damp outside the truck jerks really bad at cruising speeds.
It seems to smooth out if I stomp on the gas.
Getting a code for random multiple cylinder misfire.

I have checked the dis cap and it seemed fine.

Any help would be great thanks ahead.