If the device is mains powered there's a 50/60Hz clock on the transformer secondary
the way old LED digital clocks used to keep time from mains signal.
If you are looking at that with a pic that has an independent clock the results will vary.

If you can run an ADC from internal RC clock
Code:
DEFINE ADC_CLOCK 3
and the pic from xtal, and look at any clock or output signal
generated by the pic program with ADC the results will differ as clocks lose sync.

I would try the resistor Cap network first (timing cap discharge via resistor).