I had a similar problem. With mine, which has under 30 miles and under 20 hours on it, It was hard to start, ran rough, and until it was warmed up, if I gave it gas I would hear a pop and it would stall. The plug was carbon fouled, as well as the oxygen sensor and the tailpipe was loaded with carbon buildup. Cleaned it all up and did the same thing with replacing the spark plug and the problem came back within an hour or two.
Took it to my mechanic and after four days he said the gas tank had lots of water in it and that was the problem. He cleaned the tank, put fresh gas in it and so far, it's running like a champ.
Hope this helps ...