could be its heating up the starter sylenoid? sounds like what my dads tow truck did run it till noon shut it off try to fire it back up turns over slow soemtimes jsut clicks acted like battery was dead come to find out the starter and sylenoid were getting cooking hot and after it cooled off for a few minutes it would fire right back up. so maybe wrap the starter try to find a way to keep it cool maybe?.........scooter