Changing between the pc and the external power source the reading differs with +- 0.3V.
What do you mean by this. The range is 0 to 1023 for reading an analog pin. You are using the internal ~1.1 volt reference. What are you measuring with the analog pin ? Another stable voltage reference of less that 1.1 volts which is independent of the Arduino power supply, or are your measuing a voltage which is dependent on the Arduino Vcc (say a potential divider between Vcc and ground) ?