What is causing fluctuation in speed on Proform 545S, model 831.294252?

No matter what speed I have set, at times the speed will slow down for about 10 seconds, then it goes back to normal. This occurs both when on the treadmill and when I step off it (to see if it still happens when no one is on it). It's not as noticeable when using the higher speeds. While the motor doesn't make any unusual noises, you can hear when the motor slows a bit and then speeds up again. The belt doesn't stop. Treadmill is about 5 years hold. Nothing has ever been replaced on it. Used for light walking.