when the set up is 1/2 way or higher the main speakers go off the sub still plays the Q is where would you start testing to see if it is the amp or the deck ?
if i restart the car i works go back up to where i had it or about there it stops i also have a power off pop just when the deck powes off

 

I would disconnect all the speakers from the amp. Then, one at a time, try each speaker on it's own on the amp and see if it works fine. Usually you will find one speaker line that is the culprit and then you can move onto determining if it's a bad speaker, a grounded or shorted wire, or whatever.

I did have one time a 4channel amp that even after I had found and fixed the offending speaker, it would still shut off if all 4 speakers were connected to it. Any combination of 3 were fine but the 4th shut it off. I assumed the power supply had been compromised by the bad speaker and exchanged the amp as well. After that, it was all fine.

 

Check your ground to the deck......and amp.