carsey

Can you tell us where its grounded and the condition of the ground? Is it bare metal and not covered with paint?
Can you get us a picture?

Have you set you amp up correctly for the sub?

This is done by:
Turning volume on headunit up to 3/4.
Then adjusting the levels on the amp until the sub just starts to distort. (you can usually tell). Once its started to distort, turn it back 1/8-1/4 turn for the optimum sound.

If its turning off after 15minutes, then my guess is overheating. This is caused by the gain (bass boost or whatever you want to call it) is set too high.