Aside from all the good suggestions you've already gotten, or maybe even
before you try them, you might want to take the starter out and clean the
hell out of it and it's surroundings- every part you get to. Open it up and
clean inside if you can get it apart; check out the condition of the
internals. It's probably never been done and if so it needs it, or put
another way, you can do all of those other suggested things and still have
problems because you haven't done these things. Clean the bellhousing where
it bolts up. Check the cables and replace the connectors if necessary; clean
the connections of the transmission ground strap and replace it if it looks
like it might need it, or just for GP while you're in there. Put dielectric
grease on the connections.
I was having ongoing issues with my auto starter and it finally just
wouldn't crank anymore. It left me at the post office and I had to get it
towed. Looking through the archives and getting some feedback from the list
all pointed me to doing this step first, and it cured what was ailing it. I
was ready to drop some cash on another one and I would've just wasted my
money. This was five years ago, btw and it's still working properly, but I
may go through it again while doing some upcoming work as a preventive
maintenance measure.

Cya,
Robert