I have a Craftsman 48" snow thrower about 5 years old. Started to have a problem.....if it's in a forward gear I can go several feet and it starts to slow down eventually moving into reverse. Any forward gear I put it in, whether blowing snow or moving on dry pavement, it will start to slow down and go into reverse. I can see the gear linkage at the end of the cable where it enters the housing actually move on it's own causing the cable to slacken. This means the friction disc (rubber disc) is moving across the drive wheel on it's own without manually moving the shifter control lever. If I have it in 6th gear it will move all the way through the lower gears and into reverse. When I let go of the drive clutch handle (which separates the wheel from the disc) the shift linkage will move back to the selected speed quickly but as soon as I depress the handle again it will go a few feet and start to slow down again into reverse. I haven't taken the housing cover off yet (want to wait til the end of winter) but suspect the wheel has a spiral groove worn into it or grease/dirt buildup on it. Thoughts why this shift issue is happening other than my idea? Thanks!