Yes, these pulses were from ADC. If turn ADC off, pulses are gone.

Problem is solved, but it was weird.

Since PIC has two Vss pins, I was using one Vss for GND, and to other Vss pin I've tied LCD gnd pin. So current going trough the PIC was causing the troubles. After I've added jumper to feed Vss directly to LCD, problem had gone and linearity improved.