Lateral balance will effect harriers even if the plane is in trim. I am not saying it is the problem, but per your symptoms it very well could be. Its not uncommon for a plane that is out of lateral balance to fly very well with virtually no trim adjustments, but when you pull hard on the elevator you accentuate the problem . Tor asked a good question. How does your plane do in an elevator maneuver? If it rocks in a harrier it should REALLY rock in an elevator.

Is the plane noseheavy?
How much does the plane weigh?