[MagicRat]

You might simply have a worn starter motor or soleniod.

The next time it happens, try tapping on the starter motor itself with a hammer, then try the key. It's even better to have a buddy turn the key while you are tapping.

If that does not make a difference, try moving the gear shifter through all the gears while turning the key. If this makes a difference, your neutral safety switch may be bad.