Try adding an LED and make it blink at a steady rate. Give the PIC a heart beat.

Does the LED blink as expected?

Also try the ADC at 8 bit resolution.

Let us know what happens.

When you tried a capacitor on the ADC, did you have it from the ADC pin to ground? Was the value at leat 1uf ? l use 22uf myself.

Like skimask said, post a schematic. A picture of you setup might help too.