Airfile!! My Beta PC9 Goes uncontrollable @ 260kts

Gudday, As per the toipic above, I have a Problem that I cannot seem to solve! I am currently building a Pilatus PC9/A (FSDS) and have got all other flight dynamics down "pat" except for the Max level Speed. BUT!! When accelerate to 260kts IAS, the aircraft's elevator start's (for want of a better phrase) fluttering. As one uses a M$ Force Feedback 2 Joystick, the elevator motion is very "violent". If the speed is not reduced the aircraft will either dive or climb severly without recovering. Can anyone tell me what if any setting in the airfile that can be changed? Any Help will be greatly appreciated. P.S. The aircraft is being built for an Virtual Aerobatic Team based upon the Royal Australian Air Force's "Roulettes". See http://www.aussim.com for more info. (excuse the plug!!)