My 325 with 18HP V-Twin Kawasaki engine starts and runs well when cold. After about 30 minutes of use the engine will bog down under load and cannot recover. Especially when I engage the PTO but also when I try to accelerate quickly. So I think that eliminates the deck as the issue, but I have inspected the deck. All spindles and pulleys turn freely. I have replaced the carburetor, fuel filter, and air filter. Could this be an ignition coil issue? Can the coil start to break down after warm up? If not, what else could it be?