suddenly slows after 20 min operation?

Run your treadmill at your speed with no one on it for 40 minutes If it doesn't slow down you have a worn belt and deck or a belt that needs lube.
I have a video explaining this if you need it.
Maintaining Your Home Treadmill Revised at,
http://www.mindbites.com/person/214-Mark
If lubing doesn't help you may have other issues or a worn belt and deck.
E-mail me if you need to

Mark Peterson
March 2008