hi
first, try a .1uf cap from the AREF pin to ground. That will filter the analog reference voltage.
Second, you should look at the circuit design, as 10% variation corresponds to .5v, which is way too high. The datasheet says something like 3 bits maximum error, which corresponds to 8/1023, which equals 40 millivolts. That's a .7% maximum error. If you are getting 5 or 10% error, there is something very wrong.
So there is very likely something wrong with your circuitry. Is your power supply well filtered? Do you have noisy components like motors, relays or solenoids in the circuit? Do you have a filter cap near the voltage dividers for the sensors?
Are your sensors forming a proper voltage divider? With that kind of variation, it sounds like the analog input pins are floating at some point. A simple test would be to set up a fixed-resistor voltage divider on an analog pin, then measure the voltage you get repeatedly.
D