I have a 1997 Dodge Ram 1500 Pickup with a factory AM/FM Cassette Radio and it will turn on and work normally then after a few minutes the radio will lose signal to all stations on AM and FM. Rest of the Radio still works just no signal. If you hit a bump the radio will sometimes again gain a signal and other times not again rest of the radio works. It will turn on, display lights up and there is sound just sometimes no signal. What could be the problem?