Check out this page:  http://type2.com/bartnik/starter.htm

I installed the Bosch relay kit on my now sold '80, but on my '85, by just
getting under there and cleaning all electrical contacts on the starter and
the wires and spraying with a battery terminal corrosion preventer has
resulted in flawless starting ever since.

Basically, the current has to travel pretty far, so any weakness in
conductivity translates into intermittent starting failure.  When all is
working like new, it works fine, so the relay is probably not necessary, but
might give the starter a little more juice, especially in the winter.

HTH,

John Haddad