I have a 27hp vertical shaft kohler command on a husqvarna zero turn that sputters under load at full throttle. It really slows down, not just an occasional miss. I can back it to about half throttle with the blades on and go easy with the hydros and it will maintain speed. Oil fine. Gas in tanks, valve open. Runs the same with or without air filter. Air filter gauge shows ok. I checked the spark plugs, one was blacker then the other; gapped and replaced both. Checked coil gap on flywheel (when doing a previous repair I gapped it to about a business card then found out it had to be precise on this model. regapped it to spec using two feeler gauges). Still has almost complete loss of power at high speed or when under load. Any ideas??