Read the Errata Datasheet :-(

http://ww1.microchip.com/downloads/e...Doc/80437B.pdf

1. Module: ADC (Analog-to-Digital
Converter)
ADC conversion on AN3/OSC2 will have large INL
error up to approximately 8 LSb.
Work around
None for the AN3 pin. For better accuracy, use
another analog pin.

This PIC is a Big Bug Chip !

Regards,
Ralf