1990 GMC C1500 when it rains -it runs rough, no power!!!!

hi there could you help? my truck is a 1990 gmc 4x4 1500 with 2930000 kms on it. This started when i would go through a carwash, now it happens when it rains. So i replaced the sparkplugs, wires,cap and rotor- still does it? If and when i can drive it when it gets hot it runs o.k. and after a couple of days of driving it will be a little better but sometimes when i start it up the idle goes up and down then stalls? and again once its warm it will idle, put in gear runs rough- bad take off? CAN YOU HELP????