A car that old there could be a lot of thing going on it. The first thing I would do is try either cleaning or replacing all of your ground wires on the engine. Had a problem with my car a few months ago, car would have a hard time turning on and would die while i was driving. Brought a new battery, changed altenator, starter. None of that worked. Change a few grounds and now the car starts up every time. Plus it cheaper to start off with cleaning wires then buying parts you dont need and cant return