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?
5.
Weather. 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!

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120th Fighter Wing, Montana Air National Guard