Pacemaster Silver Select Slowing on its Own

After many years of great performance, our Pacemaster Silver Select (purchased 2005) has suddenly started slowing on its own. Once I am up to a certain walking speed, say 3.3 mph, if I take my hands off the handlebars, the belt starts to slow and the speed indicator on the display starts flashing. Trying to walk faster doesn't help, unless I put my hands back on the bars and push a little. If I keep my hands on and focus on "pushing" slightly with my steps, I can keep it at the set speed, but as soon as I let up or take my hands off the bars, the belt slows and the speed display flashes. I have tried (in the order recommended in the manual), tightening the drive belt (up to three half turns clockwise, per the manual) and the tread belt (again, up to three half turns clockwise), but no change.