86 305 check engine light/running poor

I have an 86 el camino, it has a 305. I recently replaced the alternator. It was not charging batteries properly. It died on me a couple times, each time it ran crappy for a little bit then died. Before dieing i would get a check engine light. After replacing the battery with a charged one it started and ran fine. This last time it happened the light remains on and it starts hard and doesnt want to stay running. It has a new alternator on it. Any ideas what could be wrong? I would appreciate any ideas/advice very much