Sorry to hear that it's not working correctly. If you have the plug-in GUI open while you automate the parameter, does it visibly change, or snap to Gb? And it sounds like the system only freezes when you open the plug-in after you automate parameters, never at other times?
As far as the dropdown menus poppping up behind the plug-in, do you have the "Plug-in Editors Always On Top" feature in Cubase enabled (Preferences->VST->VST Plug-ins)? Turning that off will likely solve the issue.