I have a tractor with a 20hp kohler command vertical shaft engine in it. All of a sudden at the end of last year it developed a problem. It starts and runs at high and low idle beautifullly, but after running for anywhere from 10minutes to 30 minutes it will immediately die, like someone turned off the key. It doesn't peter out, putt, or diesel as it dies. Then it refuses to start for at least 12 hours, but then it starts like a champ and the whole process repeats itself. Help, any suggestions please. I have replaced/gapped the plugs, changed the spark generator, replaced the fuel filter, replaced/changed the oil, checked wires, etc. What am I overlooking?