[link=http://www.rcuniverse.com/forum/Engine_Problem/m_1764767/tm.htm]Click here[/link] and read the 3rd post in the thread. It may help you. It sounds like your High speed needs to be leaned out.

With the plane on the ground open it up wide open. then pinch the fuel line. if the engine picks up a lot of rpm before it dies you are too rich.