Jim,

Check the positive battery lead to the starter. Make sure the connections are tight and clean on the starter end and the battery clamp/terminal is clean on the other end. I'd remove both the positive and negative connections to the battery ... clean the terminals and inside of the clamps. Scotchbrite will clean them up or you can get a terminal cleaner tool at an auto store for a few of bucks. Once you clean the battery connections ... leave them off until you clean and tightened the positive lead connection at the starter (located on the driver's side of the block below the instake manifold.)You may want to apply some anti-corrosion spray to the battery connections and the starter lead. Make sure you get + to + and - to - connections right at the battery when you hook them back up. If the connection from the battery + terminal is loose or corroded it can cause intermitant starting problems like the one you've descibed. Don't mess with the starter connection unless it's disconnected from the battery or you may get a real shock. Hopes this is of some help.
Good Luck.