Starter question..

Its a 72' dodge dart with a 318, and the starter always works when the car is cold, but as it starts getting warmer outside and I drive it over a distance for some time when the car is warm, I get nothing when I turn the key when I go to shut it off and turn it back on. The starter doesnt even attempt to turn it over until the car cools down some.
So after about 30 minutes to an hour the car will start up again. Its an aftermarket one, it was replaced about 2 years ago. all the connections to it seems fine and there on there securely. Is it just going out? or is there something else wrong?