Maybe needle settings but if it idles good and transitions good it's already adjusted, also maybe diaphragm pump check valve in the carburetor may be worn a little, I have experienced that same problem before. check this thread out

http://www.rcuniverse.com/forum/m_7975257/tm.htm

It would surge just in the mid ranges Idle was good and top end was good. rebuilding the pump in the carburetor fixed the problem. see pictures.