Hi,
How are you ?
Run the treadmill without walking on it. If it does the same thing, you could have dust and lint on your speed sensor.
If it runs fine then speeds up and slows only when walking you could have a belt/deck issue or a belt and deck that needs lube.
I have a video on treadmill maintenance here,
http://www.mindbites.com/lesson/377-maintaining-your-home-treadmill-revised
My main page.
http://www.mindbites.com/person/214-Mark.
you an still e-mail me for help also.
Try this and let me know what you find.
Yours In Service
Mark Peterson
Mark Peterson, May 2008