I try to check it but I miss the odd topic, does your head unit have the 16pin to ISO harness? I had a problem before now with a Sony harness, the 16pin connector a couple of the small connectors in it would give intermittant connection, in my case the sound would cut to one or more speakers when driving and sometimes the unit would power off, swapped harness over and it was sorted, worth a check?
Edit: other than that could be internal fault where connector is soldered to pcb or some other internal fault, hard to say without inspection, check the harness as above first, is the unit in warranty?