Strange..Can you just test the ADC pin voltage when the PIC IC is not in the circuit and I guess that's only possible if you have used IC base for PIC..I guess you have used that. I can't understand why changing AC voltage makes things non-linear in a voltage divider!!I have tested the hardware removing the 3.3uf cap after the 10k series resistor and adding a 1uf can at AN0, no improvement
Have you checked voltage immediately after bridge? Is that change linearly or not? I might then think about sense transformer..