Sounds like your treadmill needs to be re-calibrated.
Plug in your treadmill, do not stand on the walking belt. Hold down the "Stop" and "Speed +" key and insert the safety key or magnet, let go of the two keys. Now press "Stop" key once. This puts you into the level 2 of calibration. Press the "Speed +" key slowly until you see 85 on the console, (at this point the display is going to show the percentage of motor usage and also the actual walking belt speed) at 85% motor usage your top speed should be between 10 and 10.5 MPH or (of 12 and 12.5) depending on the treadmill. You must know your treadmills top speed. (Important the motor may be damaged if you set the maximum speed higher than the maximum speed that your treadmill is made for). If the speed is higher than 10.5 or 12.5 then you need to turn the potentiometer down to the correct level. If the speed it to low you need to turn the potentiometer higher. Once the speed is correct you are done calibrating the speed. Press the (-) speed key to lower the percentage to (0). Next press the incline (+) button, this will make the incline motor go all the way to the maximum incline and come all the way down to the level position. Then remove the safety key and the treadmill will reset into the normal manual mode, when you re-insert the safety key.
Don't worry about the MPH part of the instructions just go thru all the steps and the important one for your situations is the incline motor being recalibrated.
Hope this helps,