There are lots of things that can contribute to an amplifiers overheating. Too many to list here.
What impedence are you running? What guage power wire? What guage ground where? Where is the ground wire connected? Have you checked for a voltage drop? Is the amp under a seat or pinched between box and seat? What kind of airflow are you getting? Are the battery terminals corroded? Etc....

Get an installer to take a quick look at it and get some feedback from him.