Is the motor changing speeds or just the fan blade? The reason I asked was because when my rig was new my fan was acting up and then it just quit. Turns out the set screw holding the fan blade on was loose. Able to tighten and has worked ever since.
If the motor is speeding up and slowing down I wouuld suspect a loose connection. Call MAX Fan they have great service and if it is a common problem with the fan they are probably aware of it and will probably send you a new one.
2014 GMC Duramax 4X4 DRW Crew
2015 DRV Tradition