1997 Subaru Legacy dying in gear.

Hello,have a 97 Legacy that this problem just
started.Went out to go to the store,battery dead(3
month old battery). Had neighbor jump me off,let it
run awhile to charge,then headed out. Made it
abou9a mile down the road and it started spitting
and sputtering like it was out of gas(3/4 tank).
Now the funny part. Jumped off again, started
instantly,at idle runs perfectly and never dies. BUT
as soon as I put it in gear and give it gas,it dies
again and the battery is stone dead. Jump off
again, at idle give it gas it runs fine,put it in
gear,dead. Any idea what would cause this? I know
nothing about  Subaru's, older cars and trucks I
have no problem working on lol.