My truck recently started to exhibit a somewhat erratic idle when it's cold and damp outside.

The headlights intensity will diminish, as the engine nearly stalls. If I rev it a bit, a couple of times, it helps, and when the engine is hot, it doens't do it at all.

What can be causing this?

I'm planning on changing the plugs soon, BTW.

 

Well, it could be several things. Have the belts been changed lately? Sometimes worn belts can affect the performance of an engine that is cold still. Also, if a belt is slipping because of the cold and damp (such as the alternator belt) then that can affect the truck's electrical system.

Also, doing plugs will be a good idea but I know I seem like a salesman for this stuff, but Seafoam works. Allow Seafoam to be sucked through a vacuum line (brake booster, PCV or something like it) and let it sit in the top end of the engine for a little while. Tons of info on Seafoam in this forum. I used it on my Cobra a couple of times and it made it run better too. But when I did it with this Toyota motor it just ran like new!

Also, check your air filter too. Just do a run around the truck and make sure everything seems in order. No pinched wires, etc.