I have a rear wheel drive craftsman lawn mower. Only about 2 or 3 years old. The rear wheel on the right side of the mower will intermitently engage and help push the mower.I checked the inside of the wheel thinking that the gear was stripped but was not. Even put a new wheel on that side to see if it would engage better. If I kick that wheel seems to engage after that for a few seconds and then goes out.. Any ideas?