ProForm 2500 slips occasionally erratically?

Re: ProForm 2500

the driven roller screeches, the belt slips for +/- 6 inches travel. The problem is intermittent, not full time. I have looked at the drive belt connected to the motor-- it is tight and does not slip. With power off there is no apparent slippage. The driven roller has something like a one way clutch on the end with the belt. It slips backward (from normal direction. This treadmill is relatively low mileage. Questions: what is wrong? Is it worth repair?