Need help! The car has the original speedo in it, however over the weekend it began working "when it wants to". Some times it track as it should, other times it rests on 0. Once it goes to zero, it stays there until the car is shut down and restarted, at which time it operates fine for a while before repeating the process. Is it time to replace the speedo head, or is there a relay somewhere that may be the cause? Any suggestion greatly appreciated.