amp cutting in and out but not turning off and on

okay so im running a crossfire bmf 1700 and i blew one of my subs so i took it out tried to sell it no one bought it so i just put it back on some spare subs, this is where the problems began. The amp was working flawlessly when i took it out, put it in a box, took it out of the box and back in the truck and now its messed up. i can be sitting still and the subs will play and then shut off for no reason then i can tap the amp and it will play again. also at low volumes or with everything muted the subs make a super high pitch sound (sometimes). i pulled it out today and as far as i can tell nothing is hurt in the amp but im no expert either. So if you have any help it would be greatly appreciated.