I agree, this looks like numerical oscillation. The parameters I typically set are gmin and abstol. I did a search of the term reltol in the "ngspice-37-manual.pdf" file for the term reltol and I notice two other terms called "diff_reltol" and "diff_abstol" which I am not familiar with. You may want to tighten up all these parameters and see how they affect the numerical oscillation. If you don't have the "ngspice manual" for the version of ngspice you are running, I strongly suggest that you download it.