speed reports inconsistent Maximum, current speed and elevations

For over the last year my Explorer 2 has been degrading.  Occasionally I would get reports of over 100 MPH maximum speed.  then the elevation tracker started to get wrong results.  And at the end of last year the average speed and total distance was about 20% below the real value.  This knowledge was determined with historical knowledge of recurring routes and comparison with my GPS watch (BIP 6).