[Herndog]

I have a Cub Cadet 1250 with a recently rebuilt Kohler 301 engine. The engine runs fine for about 30-35 minutes, then starts running poorly with such a loss in power that I just have to shut it down in the middle of mowing our yard. After about 45 minutes of a cool down I start it back up and finish the yard. I have replaced the ignition coil thinking it was getting hot and then causing spark issues, but the problem still exists. (I have also replaced the spark plug as well.) I do not think it is a fuel problem since this scenario has repeated itself several times: run great for 30 minutes, then runs terribly. Any ideas about what to try next?