This problem is a common artifact in AFM. It is due to the laser light spilling over the edge of the cantilever, being reflected off the sample, and travelling back up towards the photodetector. The reflected light interferes with light reflected from the cantilever, causing typical wave-like oscillations in the image (oscillations in the fast scan axis, or if you look at the images, bands running near-vertically in the slow scan axis) or in the zero force line of force curves.

Typically, the wavelength of these oscillations is two wavelengths of the laser i.e. it is about 1.3 micrometers for a red laser. It is more common with reflective samples, with high coherence lasers, with narrow cantilevers, and when the laser alignment is not perfect. The typical way to fix it is to re-align the laser, trying to make sure the spot is right in the middle of the cantilever.
Newer instruments often have low coherence lasers to reduce this problem. However, with some instrument/cantilever/sample combinations it is very hard to avoid.