630's are old enough that I'd start looking at capacitors going bad. Not having a schematic in front of me, I don't know what sort of components are in the path of the power input that could crap out and do this. The other thing I'd look at, considering it's usage, would be dust, corrosion, or just some water intrusion into the radio.
Have you opened it up to take a look? Someone been washing out the inside of the cab with a pressure washer? Has vibration cause the circuit board to work lose, components to come lose, etc? Probably the first thing I'd do would be to open it up and do a visual inspection.