Craftsman garage door opener works sporatically
I have a Craftsman garage door opener model 139.53606. When the remote is pushed to open the garage door the motor runs but the chain belt either doesn't move or just moves slightly. If I try to move the chain manually, it will move by itself about 6" and stop. When I move the chain again, the same thing will happen. When I took off the garage door opener cover, there were shredded pieces of plastic in the bottom. But when I looked at the gears and drives, they looked OK. I need suggestions on whet to do next. If I need to replace any of the gears, is that something that is difficult to do?