Any thoughts on this:

This problem started occuring in my truck a few months ago. When the sound is turned to moderate or high volume, the amp goes into "safety mode" after a minute or so of playing, basically shutting off temporarily and then coming back on again about 10 seconds later. Originally this didn't happen, I went a year or more without having this issue, this developed recently. I thought it was a problem with the amp so I sent it in for repair. I the meantime I replaced the amp with a NEW Alpine amp and the same issue occured immediately. I checked the power leads to the amp from the battery and found no problems. I also disconnected the speakers one at a time while playing high volume and the amp still shut off. Additionally, I wired each speaker out of phase to see if that changed anything, still nothing. If anything, the NEW amp seems to be a bit more sensitive than the OLD amp was and shuts off easier. Any ideas?