Pushing snow recently and when backing up, machine came to a complete stop like something locked up. am able to move forward then backwards, sometimes it will lock up again but can still move forward. other times i can move forward then it will back up with no problem. machine is moving considerly slower also. i have gotten underneath and nothing is interfering, filter has been changed recently. not sure if the diff lock is engaging ) foot is off the pedal) or what is happening. Any ideas?