Could be an under voltage issue or even a RFI issue. When the engine is running and using non-resistive spark plug wires, it produces RFI simply from the sparks plugs firing. And that can travel on the DC voltage to the stereo. Or it can even be broadcasted to the stereo. I don't think it is an over voltage issue because those stereos have their own voltage regulation circuits inside. A low voltage could be harmful, but I don't see a high voltage doing it. You can install a filter suppression circuit to help eliminate the DC sporadic issues on the power supply lines, but if it is picking up RFI over the air, it isn't going to fix it. You can determine which is causing the problem by using the CD or some other non-radio sound through it. It the problem still exists. It is from the power lines. If it is clear, the problem is RFI over the air interference with the radio section... JMHO!