'83 300 TD

I just put this car on the road, it had been sitting behind a friend's barn for a couple of years, so I expected some issues...here's the first.

The other day I turned the key & nothing happened. I checked the battery, it was fine. I got under the car & tapped the starter with a mallot, she started up again. I drove her around for a couple of days, each start growing weaker until she gave up the ghost. Then I replaced the starter ( I happened to have one from an old car). She fired up & ran fine for a few days until one morning I turned the key and she sputtered meekly and died again! Checked the battery again, fine. At this point I hoped it was a connection problem and replaced the wires from the battery to the starter, alas with no improvement.

That's where I'm at. What could fry 2 starters? Or more optimistically, what else might be wrong (assuming the starter is ok)?

Any input much appreciated...Thanks!
Greenjed