With a starter struggling to turn the engine it also might not start. But This is something you should HEAR? Is it turning much quicker with a starter pack?

In case your starter really is slow, check the mass connection between battery and starter. Wires - even the thicker ones - can corrode or break on cars as old as yours. And think about a new starter. A more modern one usually works better, even with a weaker battery.

And check the connector on top of the distributor. Had a problem with mine in my XR2 over twenty years ago. The connector was broken inside and the contacts weren't held in place anymore.