I have a John Deere Z425 zero turn mower with a Briggs & Stratton engine. The engine is model: 44R677, type: 0015G1, code: 140120YG. The mower runs perfectly fine for about 15 minutes then will sporadically begin missing/stuttering. It never dies but the longer I mow the more frequent the missing/stuttering becomes. It seems to be more frequent when engine is under load such as increasing speed after a turn or going up a hill. The increased load of mowing taller grass seemingly has no effect. Here is what I have tried so far with no success (some is just routine maintenance but I figured I would include it anyway). Changed oil, replaced air cleaner, replaced fuel filter, replaced spark plugs, checked/adjusted valve clearance, replaced vacuum hose running from valve cover to fuel pump, verified that hole in gas cap is clear and even tried loosening gas cap for a while. It only seems to be getting worse. I have no idea what else to try. Anybody have any suggestions? Thanks!