sounds like your engine is running lean. i would be interested to know what brand of engine you are using. until then...back off (richen) the high speed needle a few clicks...start the engine. hold the plane with its nose up in the air, give it full throttle, and then adjust the needle just to the point to where it starts to lean out. it should be set slightly rich and dont worry...it will lean out more when you get it in the air. if while you have the nose in the air and the engine still sags....then richen the needle more until the sag stops...then go fly the beast.
have fun...