So very glad I popped in to see if others were having this problem! Brand new MBP i5 with Mountain Lion (clean install), clean install of FTMM2. Downloaded my tree and synced. Then started having problems. Runs at 102-104%, hangs, have now lost syncing capabilities. On the site it was telling me the tree was still syncing, on my mac it says its not synced. Cancelled the sync from the site and still nothing.
I have discovered the best way to run the program is to shut down ALL network activity and programs not required, but I still get hangs. Very frustrating!
Disk permissions are okay, running verify disk at the moment.