Car idles really low and stalls when I put it in reverse

My car ran fine, it wanted to stall once or twice a month when my foot was on the gas ran it real hard on the highway. Yesterday the alternator burnt up and I went and traded at a salvage yard for one from a Devill. I put it in and it ran good for an hour or so. I parked it went to go get in to drive started idles for a little put it in reverse the car stalls. If I am not on the gas not so much in drive looked down the dash said the battery is not charging. The connection was loose I tightened it and it ran good in reverse and drive still idles low, but the connection went loose again. I put a new nut on it it is staying right, but my car still stalls in reverse not so much drive.