I really need to watch you sailing to tell, but this is my guess (its a common problem).
You are leaning back towards the tail with your front leg straight and your back leg bent, with your shoulders twisted in the direction you are going. This makes it difficult to sheet in, so you pull the rig down and open, thus loading your rear leg. This decreases pressure on the mast foot in gusts, the board will pitch up and down more, making control difficult.
look at this picture of my old malaysian sailing buddy Leo Leow, at one time my closest rival for the m'sian championship...I'll explain the stance below the picture:

Leo Leow, Hypersonic + Retro 2003
His knees, hips and shoulders are all facing the board. If you drew a line through his knees, hips and shoulders; that line would be parallel to the centre line of the board. His head is looking over his shoulder at where he is going.
His entire body weight is committed to the harness, arms are relaxed and staight, upper body leaning way out to counterbalance and stabilise the rig. Legs have just a slight bend to them.
He is hold the sail as upright as he can while closing the gap between the foot of the sail and the deck of the board. You can clearly see the twist in the sail, this enables him to keep control even in big gusts.
Watch any of the faster local sailors, the stance will be very similar to this. Its very balanced and smooth power, nothing brutal or rough.
[ 05. January 2003, 08:54 AM: Message edited by: sukhdev ]