I have ckecked that the periodic behaviour of the bias-level is the same over the whole area of the CCD. I have chosen night JD2455745 for the investigation because all the dark-frames have the same exposure time (60sec).
I found the mean of five small areas of the CCD and plotted them together with the mean value obtained from the full area of the CCD as a function of time since first frame. The selected areas represent the four corners of the CCD (1 pixel away from the edge) and the center of the CCD. I therefore expected that each area would have its own mean level, but the same period and amplitude as the whole frame.
I have tried this for two different area sizes, 8×8 pixels and 16×16 pixels. The periodic behaviour can be seen in both cases, but it is quite muddy for the small area size. The above image shows the 16×16 case. The red stars are the mean values of the whole frame and each colour represent one of the five 16×16 areas. All the five areas follow the same period with roughly the same amplitude as the whole CCD.
From this I conclude that it is safe to assume the periodic behaviour is a shift in the bias-level of the whole CCD area. It is likely to have to do with the thermostatic control of the temperature of the CCD.