Cubase 6.5 freezing on close or quit

SYMPTOMS:
All working fine until I close project or quit Cubase, then it freezes and goes into an endless spin ball and I have to force quit - Every time.
I just received a really amateur Google/bad English translated reply from Steinberg support that was of no help at all, so I’m hoping that there might be someone out there a bit more professional who can help me.
The support reply basically said that my version (6.5.5.176) is not compatible with my OS (OSX 10.9.5) and that I will have problems unless I go back to a 6.5.5 supported OS.
However, my system had been running fine for over a year with no problems at all under the current OS.
I’m not sure what has caused the change, but I’m suspecting that an app called MacKeeper might have caused the problem.
ACTIONS ALREADY TAKEN TO TRY AND FIX:

  1. Backed up my Cubase 6 prefs file in Users/Username/Library/Preferences/Cubase 6
  2. Opened Cubase app again which built a new Cubase 6 prefs folder in the above directory.
    DIDN’T FIX PROBLEM
  3. Reinstalled Cubase 6.5 from the original installation DVD
  4. Updated Cubase to 6.5.5.176 from the updater bundle downloaded from Steinberg.
    DIDN’T FIX PROBLEM
    So although I can still work, it has become a real pain and I hope there is someone out there who can help - I would be very, very grateful!