If the battery is fully charged then check the terminals for corrosion. Next check for current drop between the battery and the starter. Check that all connections related to the starter are secure.Then I would suspect that the starter selinoid is going. You can remove the starter and have it bench tested at your favorite parts store. My local O'Reillys just got a tester that can check battery status, alternator output and starter quality by connecting to the battery. Not sure how it work though it seems accurate. Dan