Here are some ideas that come to my mind:
1. Persistant damage from an earlier flight
. Have you checked the maintenance hangar to overhaul the engines, airframe, etc.?
2. Throttle technique
. Are you moving all of the throttle levers forward at the same rate, at the same time?
3. Graphics settings too complex
. I've had aircraft become uncontrollable on take-off, detouring through the grass and ending up smacking into a building or something. When this happens I've found that reducing the autogen or AI traffic usually helps. If you are trying to fly out of a major international airport near a populated urban area--basically lots of autogen buildings, trees, custom scenery elements, AI traffic, etc--this might be the cause. Try a smaller, remote airport as a test.
4. Joystick needs to be calibrated.
How are your stock aircraft behaving?
I'd be surprised if this is the cause, but if you are using real-time or custom weather and the wind is really strong, it might be blowing you off the runway. (I've actually created wind/rain conditions so extreme that my parked Cessna was lifted and tossed through the air toward the neighboring city!
_________________I like Thunderbolts!
(Link to MySpace tribute page)120th Fighter Wing, Montana Air National Guard