Hello,
On the 18F252 there's no interrupt capabillity on PortC.

Your schematic really doesn't show up that good here but it LOOKS like you have a 10k from 5V to the "top" of each of the switches in the encoders, then the other side of each switch thru another 10k to the input with a 100nF cap to GND, is that correct?

If the above is not correct please post a proper schematic for us to see.
If the above IS correct then it's not a very good hardware design IMHO. When the switch in the ecoder is closed you have 2*10k in series to the input, decoupled by a 100nF cap, this may be fine. But when the switch is open there's no path for the capacitor to discharge except thru the fairly high input impedance of the pin. In effect the pin floats which is not a good thing for inputs.

/Henrik.