We had a similar problem on two hawks that we flew in the uK about 4-5 weeks ago. We tracked the problem to two Futaba servos and incorrect linkage set up.
When We tested out stab at full deflection you could feel a relatively strong servo Resistance. But from Neutral if you put any pressure on the stab and tried to pull up on the elevator stick it took very very little pressure to stop the stab from moving. See if this is your problem.
The only other thing it could be is the missing air brake cover. It is quite a big hole and that far back could result in it doing strange things.
we need one of those aerodynamic experts to pitch in here and solve that one.