I had this issue when the engine was hot also, finally replaced the starter with a hi-torque unit and problem never reappeared. Research "Heat Soak", not sure why this would happen with a new starter but then again...
edit: If you don't want to replace the starter again you could try this relay kit. Note I don't have any direct experience with it.
http://forums.pelicanparts.com/porsche-911-technical-forum/602587-what-does-hot-start-relay-do.html