The starter problem is most likely one of two things.
1) Poor connections, especially the ground.
2) Heat soak in your solenoid. You have a few options to help keep the heat off the starter. A really cheap option is to get a piece of insulation and some foil tape. Dissasemble the solenoid and wrap it in insulation and hold the insulation in place with a layer of the foil tape. The foil tape will help reflect heat and the insulation is... well... insulating

. Be careful to not put insulation in the area that is closest to the main body of the starter. Thin clearance there prevents you from being able to reassemble the solenoid to the starter if you wrap insulation all the way around the solenoid.
__________________
__________________________________________________ ___
Ja Whitfield