Stereo turns off at high volume... please help

Has anyone had this problem? The stereo is set up like this:

Deck is powering 4 6X9s with 50w per channel
Amp 1 is powering the two tower speakers
Amp 2 is powering the sub

This is for a friends boat so I don’t know exactly how these are hooked up but I do know this is only a 1 battery set up.
I do know that the cable to the battery looks a little weak in my opinion and he doesn’t have any “monster” cables in his set up.
Also, the ground wire from the amps and stereo are all spliced together and are grounded to the same point. We checked to see if this was the problem and made a second ground wire from the stereo to ground and the problem occurred.

If anyone else has had this problem please let me know what I need to do to fix it. This has happened on 2 of the 3 boats I have been on so I believe it is a common problem.

Thanks