My o6 xfire has done that on 1 occasion, about 100 miles ago. Has worked fine ever since then, my first quess would be the sensor, but also remember to check for oil leaks on the DD. I have heard that the speedo quitting can be a sign of major probs. I am not going to worry about it because mine was just properly serviced and is not leaking. I think the sensor is like 30 bucks.