I have a Husqvarna ZTR with a Kawasaki FH531V AS05. Crank it cold and it runs great for the first 10 minutes. After that, it starts sputtering when I put it under a load by thrusting forward quickly on the handles. The warmer it gets, the more it sputters when moving forward till I'm just crawling while trying to cut grass. If I stop the forward motion but leave the PTO engaged, it will catch back up and run okay. It is only under a load that it acts up. Doesn't take much of a load the warmer it gets. It never totally dies out. I first started noticing this about a month ago when I would engage the blades. It would act like it was going to choke down but then catch back up and be fine. Now it does it all the time once the mower is hot. I installed new plugs and fuel filter. Cleaned out the air filter. Fuel is only a week old. I also checked the kill switch under the seat to make sure the connection was clean and making good contact. I called a repair shop and described the issue. They said to replace the fuel lines as they could be softened and collapsing and cutting off flow. The fuel lines appear to be firm and it gets plenty of fuel before it gets hot. Another shop said to run some sea foam through it that the carburetor could be dirty. If that was the case, I'd expect it to act up even before it warms up. I'm leaning toward a bad ignition coil myself. I took the two coils off and cleaned them with sand paper and a wire brush. Cleaned the magnet good on the flywheel as well and re-installed and set the gap. No change in the performance of the mower. Anyone else experience this sort of issue?