I've had bad luck using any of the PORT C pins for pot measurement. They
are schmidt trigger when inputs, but the thresholds vary depending on the
other pins. I have not tracked down the exact things that do it, but it
could be the state of the other pins or the TRIS settings, but I now
avoid PORTC and any schmidt trigger pins for that use.

I've had good luck using any pins on PORT B, and it seems stable between
chips, too.

-Bob