Not exactly sure what your discribing, and I realize its hard to put it in text.
Did you mean the starter motor would not spin sometimes, or never kick at all.
Assuming you have good parts.
If it never spins, check your large cable for corrosion and condition on both ends. If that seems ok, disconnect the large wire and put a trouble light on the small wire and see if it lights every time you try a start. This will allow you to try it alot and not wear out the starter.
In the case of it spins but doesnt engage, could there be a tooth missing on the flywheel?
Often if a tooth is missing it's found where the car naturally always seems to stop and the starter often finds it. I have heard if this is true the flywheel can be unbolted and rotated a quarter turn and avoid the bad tooth.
Good Luck