When I measure the capacitance of two cables I get very volatile values if I go to lower frequencies like 500 Hz. The capacitance is about 50 nF. Why does this happen?
If this device measures capacitance by measuring the current at a given frequency, the current drops as the frequency goes down. This is due to capacitive reactance being proportional to frequency. (X = 2*pi*f*C) At low frequency, the current is very small and hard to measure reliably.
@gbarry has the right idea, plus also consider the fact that you're moving closer to mains frequencies (60 Hz, 50 Hz) and the fact that 50 pF is easily coupled into with stray fields. It looks like you're just more susceptible to the noise around you at the time that the instrument is needing to be more sensitive.