THE LAST COUPLE OF WEEKS MY SPEED0METER WHILE RUNNING 70-80 WILL DROP TO ZERO--AFTER A FEW MILES IT STARTS WORKING AGAIN--THIS HAS HAPPENED 3-4 TIMES--ANY IDEAS WHAT COULD CAUSE THIS ? ONE GOOD POINT WITH THIS PROBLEM IT KEEPS MY MILEAGE DOWN AS THE ODOMETER DOESN'T WORK EITHER