Please, please read this and give any advice you can give!!!

[AntwaanRandleEL]

ok I put in a new battery, and i got my alternator checked at 3 places and 2 out of 3 said it was good. I watched them test it. So i got a used alternator off of a buddy (its a fairly new ford alt that is in good working order) i installed it and the car starts and runs fine. The only think I notice is that at stop lights when im just sitting, the volt gauge drops down to about where the A is in normal. and this bothers me. is this normal? i do have the radio on but not loud and the heat on. when im at cruising speeds the gauge is dead in the middle. is my car maybe not charging well? as far as something other then the battery and the alternator? let me know what u think, thanks