Stalls at idle

Hey,
i recently bought my 1998 legacy outback, it has high mileage but the guy took very good car of it. It drove fine on a test drive and on the way home but I tried to start it 1 hr after getting home and it started and idled for about 4-8 secs and dies, after trying it a few times the battery died, jumped it and it would start but the idle would die. If I give it gas while idling I can get it rev steady above 3000rpm but below that it stutters and stalls, and after starting it a few times it killed the battery again so I bought a new one and it starts but wont idle. The only thing I did before it wouldn't start was replace the air filter, anyone know if theres cables or something around there that if bumped would cause this like the air mass meter or something, someone said it might be the alternator or the throttle body valve?
anyfeedback would be greatly appreciated. Thanks