Rain+stalling

I have a 91 nissan sentra and just recently ive been having this weird idle issue along with stalling. When it rains or when theres fog out side the car likes to just stall out. It happens when I come to a stop or put the clutch in to down shift. Anyone may know what is wrong? Thanks.

Sounds like moisture in the distributor cap, if its a G series motor then they are well known for it as its exposed to the front of the car.

I would replace the cap and rotor if they havn't been replaced recently, as they are also prone to cracking, which will amplify the problem.
If they resonably new, (with in the last couple of years) then a light spray with some CRC or WD40 will remove the water.