Since you have already reinstalled/updated PM a couple of times, we can very likely rule out a bad installation. As there are many of us running PM without any problem on similar environments as yours, there must be something else on your system causing this.
Perhaps/hopefully it's just a case of your PM preferences gone haywire. Go to the Library folder in your home directory (if you can't find it, press the alt key when in the "Go" menu of the finder, it'll pop up there), rename/move the com.camerabits.PhotoMechanic directory as well as all com.camerabits.*.plist files. When you next start PM again it will start “fresh”.
You will have lost all your settings, of course, but we can always try to recover them from the original preference files later. First we need to determine if this helps at all for your problem…
Hope this helps,
Hayo
