One year old Tractor/Mower runs fine for about a half hour, then engine starts sputtering and unit has to be shut down. Have done all the manual says..using 10W-30 engine oil (oil and filter changed twice with only 40 hrs total operation time, and also checked spark plug). Acts almost like vapor lock once experienced on old cars?? Continues to start fine when cold. Does anyone have a suggestion of what to look for?