I had a GFC500 installed driven by a G5 with a G500 (legacy).  I'm getting a slight porpoising of 1/2 degree up and down every +/-15 seconds in level flight smooth air. I believe that I'm also getting it during climb and descent.  The pitch change is reflected in the G5 and G500, but don't know if this is the cause or reflection of the autopilot's action.  The installer claims he can find nothing wrong.  Has anyone experienced anything of this nature?