Are those yellow things in the AC mains to PT lines crimp type butt splices? Unplug the amp and tug real hard on both side of the wires entering those butt splices. Also try pulling on the wires coming from the spade connectors on the AC mains socket. The ON/OFF nature you describe sounds like a bad connection somewhere. I'd recommend you remove the butt splices and solder/heatshrink that connection.