I haven't had that problem with mine. My elevator trim has always been none.

You might check your:

Check for "play" or binding in the linkages.

Be sure everything (like the battery) is secure in the cabin.

Balancing on Center of Gravity. With PC in-place and ready to fly except the fuel. Be sure it's not tail-heavy.

Elevator throw rate. It should only be 3/8 - 1/2 inches.

The wing should always be installed in the same place... centered and all the way back against the rear lip. At it's flight-ready weight of 8 lbs. , mine takes 16 rubberbands to keep the wing on and in-place during flight. It should not be moving at all.

http://www.vlsdynamics.com/rc/planes/us_aircore40.htm