my 18hp intek in my sears rider has developed a weird issue when it gets hot.it takes me about an hour to cut my grass and 1/2 way through the engine starts running real rough.sputtering and almost back firing.when it cools off it runs fine again.engine has new oil and spark plug and fresh gas.has the same issue with 91 octane and 87 octane.the vent hole on the gas cap is also clear. hope its not something serious,this machine is about 18 years old. thanks