Hello all, this is my first post. I am having problems with the engine idle when defrost or ac is switched on. The idle slows way down, surges back up, and hear a clicking noise in engine compartment every time this happens. At stop lights while in drive, will sometimes cut off engine completely. Any help would be greatly appreciated! (99' F-150 4.2 w/auto trans)

 

Sounds to me you may be in need of a tune up. New plugs, wires etc. I know when my truck needs a tune up the a/c will pull hard enough to almost shut it off at times. When the truck is tuned it's not a problem at all.