Samuel,
The program uses an Alternating Direction Implicit solver that can produce oscillations, although I wouldn't expect them to be so pronounced with reasonable settings (see pg. 13 of the M21HD_ScientificDoc).
What time step and grid size have you used? I would suggest cutting each in half (maybe more if they were unreasonably large to begin with) and observing the effect it has on your results, if you haven't already. If the oscillations disappear or are significantly reduced then I would conclude it was just numerical error of the solution scheme.
Regards