I dont know wat can cause this problem?

i got two 12" 250 wats a piece i had a 600 wat amp powering them for some reason i turn my car on in the morning tryed to put my music on nothing the green light was on the amp i changed the fuses on the amp nothing i know for a fact everything was hooked on properly i took the amp of took it to my buddy he gave me a bran new one i hooked it back up it worked perfectly until the next day i had the volume turn up fairly high then the subs cut out i knew it was the amp i took it off brought it back he gave me another one i hooked the third one up now every time i crank it the amp gose into protection mode but only when i crank it im really curiouse wat could cause this issue