I've got a wierd problem from a brand new 6HP Briggs & Stratton quantum engine. It runs fine for five minutes, but then sputters and dies, and can't be restarted for about half an hour. I replaced the spark plug when it first happened, and that solved it for a couple hours of use. Then it did it again, I replaced the spark plug, and it did not fix the problem. What is happening? It is a brand new engine with less than 15 hours of use.