If its the same final drive as the 350, you have clutches in each side, access covers under the seat, open up the covers and get a look, might have an oiling issue or the clutches could be worn or just out of adjustment. Look down inside and you can see where the fork presses on the clutch, the brake levers would have a bit of free play in them before clutch take up, I think its 1/2 inch at the floor.... Actually do both tracks stop pushing or just one side? It might be an issue with the engine clutch, do you have a reverser?