Make sure that the walking belt is properly adjusted according to your owner's manual. Here is a link for the owner's manual in case you need it:
. Make sure that the belt is properly aligned and not over tightened.
If you are using an extension cord, you may try running the treadmill with a shorter extension cord or with it plugged directly into the outlet.
The speed of the treadmill is detected by a reed switch and a magnet. The speed is controlled by a potentiometer on the console. I provided the specific technical information for this treadmill in the images below. If you have a volt/ohm meter and the technical ability to safely use this information, you may be able to diagnose the cause of your speed problem. You could have a problem with the potentiometer in the console if the speed intermittently speeds up and slows down on its own. In this situation, the console would normally need to be replaced. The console for this treadmill is no longer available. The treadmill would need to be replaced if the console is bad.
If you need more help, resubmit your question with additional details.