I OWN A 2004 ENVOY XL 2 WD. IT HAS 40,000 MILES ON THE ODOMETER. ON THREE DIFFERENT OCCASIONS THE SPEEDOMETER READ ZERO WHILE THE CAR WAS ABOVE 40 MPH. THE SPEEDOMETER REMAINED OFF LINE UNTIL THE NEXT TIME IT WAS STOPPED WITH TURNING OFF THE IGNITION. UPON START UP,THE SPEEDOMETER NEEDLE BEGAN TO RECOGNIZE THE ACTUAL SPEED VARIATIONS OF THE DRIVE. UNDOCUMENTED RATES OF SPEED COULD CAUSE ACCIDENTS. WHEN WILL THE NEXT TIME WHEN THIS RECURS ON THE VEHICLE? WILL THIS AND/OR COULD THIS CAUSE AN ACCIDENT? *AK