I have a 728 that after cutting longer than 30 minutes, will lose power and sounds like it is running on one cylinder. I replaced the plugs and I thought it was running better. I shut the tractor off to answer a phone call and when I restarted the tractor, the miss was back. When I shut the tractor off, there usually is one backfire. If I wait and let the tractor cool off, it will start and run fine until it gets warm again.