aircraft control problems

Currently, two of the jet type aircraft that I have in my inventory are causing problems that affect the performance of the aircraft.The aircraft in question are:Gulfstream V D/L from pmstone.comFalcon 700 D/L from avsim.comHere is what's happening. All controls are operating normally and the aircraft operates as it should. About halfway through the flight, while the aircraft is on auto-pilot, the aircraft attempts to go vertical resulting in the audio and visual stall alarm. I turn off the auto pilot only to find that I have to hold the joystick in the forward position to keep the aircraft from going vertical. As far as the gulfstream aircraft, I have the most recent panel for that aircraft. I just downloaded the falcon aircraft from the avsim library today.What I don't understand is if everything was working fine during the first half of the flight, what would have changed during the second half that would cause the aircraft to go vertical? This was not an issue when I first used the gulfstream aircraft. This problem happened immediately when I used the falcon aircraft.Can anyone help me figure out what is causing this problem and how to fix it?