How to get constant readings from hall effect sensor
Does the value change if there is nothing else connected? 
Do you have a constant supply from your PSU?


I'm not great with electronics, but since its a real device not a theoretical device I suspect you are always going to get some fluctuation.  Could you look at adding extra components to smooth the flow to the device, possibly a capacitor?  Is it possible to supply the sensor from an independent supply that you smooth and then any devices added to the Arduino would cause an affect.

Is it possible you are not supplying enough power to the Arduino?

Could you look at taking an average with a sliding window say the last 10 values?