Hi,
If everything works sound in your board and in your connections with the sensor and the sensor it self ain't broken then, i suggest the problem is caused by other things.
You see, the flactuation on these values could be due to white noise, perhaps some magnetic fields interfierence or anything could cause it. You have to consinder the square error in the measurements as well.
The best way to deal with this, is kalman filter.
I think will be a good code example in arduino forums for the kalman filter implementation.
Hope this helps.
Minas