Z-axis Acceleration offset

Hi,

I am using the MPU-6000 and have designed my own board for it.
on almost all my boards I experience an offset on the Z-axis of the accelerometer. It's always around 0.2 - 0.3 G.
you can see what I mean in this picture:
http://s18.postimage.org/afbjqx153/offset.png

the first is the Tracker completely horizontal; the second is the Tracker flipped upside down.

I have a hunch, that this offset is due to package stress.
Althoug I did take that into account designing the pcb. The ground-plane underneath the MPU is covered with soldermask; could this cause package distortion? or theere a different origin for this offset?
Has anybody else been experiencing an offset?

Thanks
Mike