Bill, usually what is happening is that you have some corrosion inside the
solenoid.  You can spend some time taking the solenoid apart, cleaning and
lubing things, and putting it back together, but in my experience this only
fixes the problem in the short term.  Buying a rebuilt starter (Bosch) is the
best solution.  Local part house rebuilds are cheaper, but I have found they
aren't worth the money that you pay for them.  Sure they have a lifetime
warranty, but after swapping out the starter for the third or fourth time in
a year, you will realize that this is not worth while.
Rebuilt Bosch starters we have for $130-135 with a $60 core charge.  These
have a one year warranty with a two year towing warranty, but these are the
longest lasting rebuilds out there.  Just my opinion.

Thanks,
Ken Wilford
John 3:16
http://www.vanagain.com
Phone: (856)-327-4936
Fax: (856)-327-2242