craftsman riding mower with a 17 hp kohler engine. It starts and runs fine, mows for about 20 minutes then starts to miss. If I keep feathering the throttle it will continue to run, but then there is no throttle control, just wide open. It then eventually dies. I took the carburator off, cleaned the bowl and put it back on. It has new oil, filter, plug, and air cleaner. I hit it with a shot of starting fluid and it starts back up and ran for about 5 minutes then died again. What the heck? Carb, bad fuel, coil, ? thanks for any information.