Adding to what Jumper said, I also face similar issues.
Usually, using 7805 or 78L05 may not provide a stable 5V.
Say it is actually between 4.98V and 5.02V. (approx 5% datasheet statement)

So, if you are using 10-bit reading there is your problem.
Eventually, Jumper's suggestion of having a high precision voltage reference is a good idea.

Another issue :
Pausing 2ms after each ADCIN command may help.