Does the hydro lever have any resistance when you go to accelerate or slow down? It could be something as simple tightening (or snugging, rather) the nut and bolt that holds the hydro lever on to add a little resistance to the lever. I'm not sure if how I said that makes sense or not. I'm sure someone will chime in and clear things up.
Edit: From the looks of the mower, you'll have to lift the hood, take off the drivers side (right) engine panel, and there should be a bolt that you can tighten to help the lever not drift. I had the same problem on mine and that solved it. Good luck!
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-Ryan 
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