Sub cuts out at high volume..?

Hi new to site, I just installed one 12inch sub with 600w amp that I recently pulled out of my old car and put in my cruze, I did this install with a PAC gm44 and system sounds great only up to 18 clicks of volume and anything higher the sub cuts outand bass is gone then soon as I turn volume down a little the sub comes back, I have a big enough power and ground wire and ground is solid with good contact, I don't believe it's a power issue cause it happens weather car is running or shut off, I've also connected another battery with jump cables and still cuts out, Any help would be appreciated.