I have a new Griz with about 100 miles on it and was on some steep hills and having to winch my way up. This took about 20 minutes. the atv was idleing the whole time, when I went to get back on and drive over a easy rise the engine started sputtering, and no power. Had to roll backwards down the rise and get a flat running start and use my speed to make it and as i reached the top it started sputtering again. Anybody had this happen before? Know what it is?