KAC 9152D amp goes into protect

Bad ground, after reading how you grounded it, I would look at it even closer. Do not assume that a bolt is a good ground. Read the "What is a proper ground" sticky for further info.

But even with a bad ground.. the symptoms would make me think that is not the only problem. With the amp working for 20 minutes and then cutting in and out every few minutes after that I would think that either the amp was overheating or the power supply was being depleted to drop below 12volts. Maybe the combination of a not-so-great ground and insufficient power would make the voltage drop lower/faster. Have you upgraded the big 3?

On another note, how good is your remote turn-on line to the amp? Are you running any other devices off this remote wire and are your connections good? I used to run too many fans and lights off this line without a relay. With everything turned on it would drop below 10 volts and all the amps would shut down.

Either way, I would change your ground and check the voltage at high volumes on the main power and on the remote line. And you might as well check the impedance theory by simply unplugging one of the speakers.

ya,  got a little confused there, but youre right i checked the calc.