Exactly the same thing happened to me on Mint (derivative of Ubuntu) with 1.6.5. The problem stopped when I upgraded to 1.6.7. I thought the new version fixed it, but since you're on 1.6.7 apparently that's not the case. (Plus, as with you, 1.6.5 was working properly for a while.)
When you reinstalled, did you also clean out the .lyx folder in your home directory (or wherever the local settings get store on a Mac)? I don't think that was the problem in my case, since the upgrade did not clear ~/.lyx, but maybe it erased something rude there that did not get erased when you reinstalled. The fresh install lost at least one of my settings (I had the View toolbar moved to the right margin), so something in ~/.lyx did get reset. /Paul
