My apologies if this has been addressed in another thread. I cannot get the L-39 to stay trimmed during flight. I can trim for straight and level and after a few seconds it will start a gentle dive that progresses and cannot be trimmed out. Also, when the elevator is pulled to a full up position it does not return to neutral but instead stops it's return at about 1/3 to 1/2 up elevator ( I noticed this on the ground after a few tail strikes during takeoff). I didn't know what was causing them but now it's obvious. When pushed to the full down position the elevator returns to neutral as it should. Any ideas, anybody?