Where is the fuel tank vertically in relation to the engine? If the tank is too low it can cause that problem. I had a similar issue with a os .46FX in a Hobbico Spectrum. I put a one way valve in the fuel line and it runs like a champ. I don't know why it works, but it does.
Here is a link from tower... I think I picked mine up from the LHS for a couple bucks...
It also makes a nice spot to re-fuel from.
[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXG850&P=ML]http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXG850&P=ML[/link]