22 HP Kawasaki Engine

I have a 12 year old EX Mark with a 22 HP kawasaki engine, twin cylinder. I have replaced several coils last year and starting out doing the same this year. I don't believe the coils are bad but something is making them go out. After about an hour of mowing it starts spitting and sputtering and loosing power. I can nurse it back to my shop by choking it. Any ideas what could be causing this?
 
Thanks, Marty