i had the same problem with a 40 extra 300 and it was the differential in the elevator. I was using one servo on a Y push rod and it would get more than a 1/4 inch off at full throw. This made it barrel roll in a tight loop. I connected the elevator haves together with a piece of piano wire. It solved that problem. I don't know if this will work on a 120 size but central hobbies sales an elevator bell crank that eliminates differential throw. If your servo are in the tail then, never mind , try a match box.