1989 Ford F-150

I have a 1989 ford f150 4wd 160k miles inline 6cyl just bought it and the guy I bought it from said it needed one of the sensors he didnt know which one it needed though. But anyway it will idle ok sometimes but sometimes it wont. It will just die and it will start right back. If I slow down to a really slow speed or gear down it will die. As long as its driving its ok. It acts like it may have something to do with the battery because if its idleing and I turn the ac on the lights dim and the battery gauge goes down and the idleing goes down but it dies either way if someone could help could u please let me know