Good to hear from CS Team at last.  Possible perhaps, but looking over the evidence posted above, can you recommend any troubleshooting steps I might take ?  BTW I have also tried completely uninstalling the CSII from P3D5HF2, (including manual deletion of the residual CS folder uninstaller leaves behind), reinstalling and launching the default startup, with no fsuipc profile.  Stab trim still runs away in P3D5HF2.  Doesn't do it with identical fresh install in P3D4.5HF3.  Nor with any other stock or add-on aircraft in either simulator.