Sony-Ghost in the machine

OK here's the situation.. 1999 Jeep grand cheroke with infinity premium sound. Deck install, 4 channel amp, sub amp, 2 12's, electronic cross over. The 4 channel was T-tapped into the factory wiring at the amp the factory amp was compleley removed. The amps were really old but they still worked great on the test bench. I dont rememer the manufacture of the amps.. The factory speakers were left stock. The subs were audio bahn. The head unit was a 2002 model sony it was the model x-plode with the full black face and controlls behind the face. At the end of the install uppon testing the system the head unit volume kept jumping to 30 after pressing the volume button or track selection. we swaped out the head unit with an brand new sony unit and it kept doing it.. I unhooked all of the amps so the head unit was the only component hooke up. 4 wires power, ground, ignition, remote. It continued to malfunction. I unhooked the original sony harness and replaced it with a new one. The problem stopped. After the customer left I took his harness and installed it on a new sony unit in our sound room, No problems. Has this ever happend to anyone else before.