analogRead values unstable

I have been using an ADXL330 accelerometer with an Arduino board and it reads the values without a problem using the analogRead funtion. The only problem is that when I leave the accelerometer perfectly still, the values of the readings change significantly, even though there is no actual acceleration.

I have tried taking averaged values (reading the accelerometer up to 100 times in a row and then averaging the sum of all of these) and yet the values still fluctuate. I have next tried to divide the averaged values by 4 in the hope that this will reduce the fluctuation, but still no luck. Does anyone have any ideas how to solve this problem? I was considering changing the A2D prescaler, but wasn't sure if that was the best idea due to the whole clock speed issue.

Any help / advice would be much appreciated.