Why does my Leisureworks 539 slow down lower than setting ?

My Leisureworks treadmill 539 has worked very well since I've had it for two years. However, recently when I reduce the speed to cool down after about 20 minutes or so of use, it tends to drop way below the setting and seems to want to stall at times. If I set the speed at 2.5mph for instance, the speed would suddenly drop to 1.8, then jump to 2.7 and other speeds in between on its own, for example. The speed then becomes sporadic. Has anyone experienced this with his/her treadmill? How do I fix it? Thanks.