Another cheap success story. You got it right in that every time the contacts make and break there is a small arc and some metal melts causing roughness on the contact surface. Eventually the contacts may not make a good electrical connection and all it takes is a little movement (2x4 shock) to cause them to make contact. Some relay contacts actually do a slight wiping action when coming together which tends to keep the contact surface clean and smooth. If the 2x4 trick works then it's time to open the case and clean the contacts. To restate something, it is best to use a file instead of sandpaper because sandpaper can leave grit embedded in the soft contact metal.
Al