Idle Problem

My 1995 Cougar V8 When Sitting And Idling Will Drop In R.p.m.s .i Can See The Voltage Gauge Drop And My Head Lights Go Dim Any Ideas.

when you put it in gear what happens. it could be the altenator starting to go. my 90 cougar did that

im going with the guy above me because i had the same problem and no lie three days after it started to do that....my alternator went out on me at a gas station so try and replace the altenator and see if thats the problem