Sputters for second before turning off & at High RPM?

This is called dieseling it means that gas in your cylinders if firing from heat of the cylinders alone. This is caused by lower octane gas which is more combustible. After you shut the engine off any uburned gas left in the cylinders will self ignite provided the engine is reasonably hot. Meaning the engine will "run-on" until all of the reamin fuel is burnt up. Try higher octane fuel or an octane booster from your local parts store or general store. This may help. If thats not the case fuel injectors could be leaking causing the "run-on" These are the only two causes I could think off. Other than extreme internal engine heat. But you would see signs of that by your gauges.