resistor drift via temp messing with ADC readings in voltage divider
The one time I have experienced a similar cyclic drift in a resistive voltage divider, one of the resistors was exposed to a varying airflow from a nearby device fan. Since my divider was initially similar to yours, the higher resistor was dissipating a lot of heat. In your case, at a full-scale of 64 volts, the 4.7k resistor dissipates around 0.8 Watts - which is a lot. 

A solution would be to use a divider consisting of 620k and 18k, thus reducing heat generation massively. If the ADC requires a lower impedance source, a single-supply rail-to-rail op-amp (e.g. OPA2192)powered from the +5V isolated supply would be used as a buffer (voltage follower).







simulate this circuit &ndash; Schematic created using CircuitLab

Why this works:

The current through the resistors is reduced, hence reducing heat generation. Thus the temperature of the resistors does not rise significantly. This ensures that differential thermal drift, and especially change in temperature due to external cooling such as airflow, has much lower impact.