The Eye Tribe

Hello, my EyeTracker seems to be somewhat inaccurate.
When using the program seen below the gaze is off all depending on where i look.



When looking at the Letter inside each button the first four lines the cursor seems to be placed correctly and often touches the Letter it self,
How ever when looking at S, T, U, Y, Z and Æ buttons
the cursor is still inside the button but starts to flicker a little.

when looking at the V, W, X, Ø, Å and blank buttons the cursor flickers a lot, and is mainly placed at the button on top, (etc. looking at W places the cursor inside the Q button and often near the Q)

I've already tried re-calibrating thé Eye Tracker, at both 9 and 16 points, at multiple différent point sample times, all giving similar results
The screen is 24" running 1920x1200
And the program is just a simple full screen windows form application (The buttons located at center of screen with size 1024x768)

Any idea to how i can fix this?

try not to move backward or forward on your chair(even by an inch), accuracy is better but it is very difficult to control these movements