Chasing a starting problem

Do your best to describe exactly what happens and when.
So far there is about 1000 possibilities, so lets narrow that down some.:D
Okay thanks, it's an LA360 in a 71 Dart Swinger with electronic ignition. It starts easily on most occasions, but it seems that after a moderate amount of driving it won't turn over at all, no loud clicking, only a really faint click can be heard. If I jump it at the starter relay it fires right up and often times goes back to working normally for a while again.