A workaround I have found is to open the MFD display as an external window (Alt-GR + click on MFD display).
Having the MFD visible (either from cockpit view or as an external window) seems to cure the AP inability (in 1.33.8.0) to maintain the course (esp when it changes) and VNAV limits.
The External MFD window can be resized (Edit: or minimized) and placed somewhere that is not obtrusive. If you close the external mfd window, the AP issues come back.