1987 Buick Skyhawk wont hold a charge

Driving home at night headlights and radio on, suddenly a mile from home headlights go dim. Get home and next day car wont start. Jump started battery car started. Drove 15 miles, shut car off wouldn't start. Jump started car and went home. Replaced alternator and battery. Car started fine got 15 or so miles down road and head lights went dim again. Car wouldnt start next day. Replaced alternator again, still having same problem. Any suggestions?