capacitor issue maybe? It stores up power to provide a stronger pulse to the noid and they do go bad.
Try replacing the capacitor, install it correctly (they have positive and negative leads, don't mix them up).
The negative lead is marked by a stripe down the side of the cap. Rubycon, Panasonic or Nichicon capacitor rated for 105 degrees C.
Radio Shack will work in a pinch.