As already suggested, first disconnect the input to determine if the problem is internal or the input. If internal, it's also possible you have a capacitor going bad in the power supply. These are usually larger elecrtolytic (can type) capacitors. You may also look at the cap to see if there's a bubble or if it's leaking. If you find a bad one, look at the value and voltage rating on the side (example: 10uF 35v) and replace. I've seen where one leg of a cap has come loose because of the vibration. I quick touch with a soldering iron may fix it. Good luck.