I have a zero turn mower w/ 24hp Kawasaki engine, it runs 20 min. then RPMs rise and fall then engine dies on it's own. I gotta wait 3-5min. it starts runs 10-15min. same result. I have changed spark plugs/ALL fuel lines/ fuel filter/fuel pump/carburetor. Most recently adjusted the valves, they were off and this got 5 hrs. straight mowing last week. Today back to the same crap! Ideas please.