I would (after disconnecting the battery) take the starter switch off the top of the starter after unscrewing the rod of course, and check it out real good. I't easy enough to do, and the switch itself is incredibly simple in function - basicall a copper strip is pushed against a copper post on the starter. There is a treated paper insulation - check that it is in the right place and not deteriated. Then clean up the post and the copper strip with some fine sandpaper to inusre a good contact without arcing. That should do it - good luck.