I've been having a problem with my 2000 F-150. When we are in gear, the RPMs will rhythmicly pulse up and down a little bit. At the same time the headlights will dim. This started right after we changed the alternator. The new alternator has tested fine at the auto parts store. We have changed the PCV valve, cleaned the IAC, and checked for vacuum leaks. Any ideas what could be the problem? Thanks for your help.

 

head light will dim on low rpm. I would remove and clean the TB. If you tap on the IAC with a screw driver handle not too hard, see if that changes the rpm, if it does, then replace it.