I have the same problem with my 84 300TD. Some times it will start from the key, and some times I have to jump it on the box on the right fender. I also get a loud click when I turn the key and it does not start. I have noticed that it will start more reliably when cold then when hot.

In my case it is not the selector bushings, and I don't think it is the neutral safety switch. The fact that the car starts when jumped suggests it is not the starter either.

W.