Power supplies under load can vary slightly as they adjust to the load.You could also try to use a sketch that has a running average (best bet)One such example HEREAnother trick passed to me on these forums was to take a reading and ignore it then take the next reading.In my case that pretty much worked for me.
Ripple on the supply?Or is it a battery?Does using the internal voltage reference improve things?
I was surprised that the ADC reading was different depending on the power supply.
I'll have to research what "internal voltage reference" means - I'm not sure how to use that in this situation.
Is it possible that the internal DAC is "broken"? Does this happen?