1988 Volvo 240 No Start

Electrical problem
1988 Volvo 240 Engine Size unknown 300000 miles

Ok, so I have a 1988 Volvo 240 DL. Recently, it hasnt been starting. I've replaced the battery, then the starter, then the alternator, in that order. Every time I replace something it will start maybe four or five times and then it will stop starting and I have to jump it. And even when I do get it started, the lights will barely come on if the radio is on or they'll go dim if I roll down the window, etc. COuld it be that, even though it's a new battery, having the wear of an old alternator and starter have drained it? Or is it something else?