Hi Bob,
Look at the starter, battery and connections. Start with the connections, only because they are easier and cheaper to deal with. Make sure everthing is tight and free from corrosion.
Have the battery tested.
Last, and probably the source of your problem, check out the starter, solenoid and relay. Internal dirt, wear and corrsion sometimes cause these issues. Since the solenoid is integral (mounted on the starter body), you may have to remove the starter to work on it. It would then be a good idea to test the whole unit at a qualified shop or just replace/rebuild it if nothing else is obvious.
I hope that helps you. Good luck.
Art