ORIGINAL: Dr1Driver
...You either have loose hinges or the torque rod between the elevator halves is loose in the balsa. Also, check if the elevator halves are lined up evenly. The rod may have twisted. Whenever I build a model with separate elevators, I try to run two servos, one to each half. IMO, the torque rod is a problem waiting to happen.
Good luck with it!
This is the most likely scenario. If you add "up" elevator and the model snaps, then you have an accelerated stall problem. If the model just roll, like you've added aileron, it's most likely one side of the elevator moving more than the other. You do need to check the interconnection between the elevator halves. You don't need to use two servos, as Dr1Driver said, if you have a well-made interconnection. Many times, though, the carry-through, be it wire or wood, is weak and allows the side with the control horn to move more than the other side. Sometimes, there's no connection at all because it's broken. You can get enough elevator control from one side to fly the plane, but try to do some aerobatics and you'll get a surprise.