My 3320 ehydro has been loosing MPH. The manual says in range "A" at 2600 RPM I should run at 3.8 MPH. I have tested range "A" and "B", both fail the test unless I run in reverse. The best I got in range "A" was 2.6 MPH. It is due for a fluid change but it is not over due or low and the air filter is clean. Could this be a calibration issue?