Personally I have used 643 to complete projects but it has some stability issues so I returned to 550 for the time being. I haven't seen "playback error" though. Can't say if 636 is better than 550 as I haven't had the time to test it against 550 (and don't plan to). Issues seem very system specific, though, so you may have a great experience with 636/643.
ok, Then do go ahead and update to version 633 and see if the problem persists..... Still im wondering if its due to a problem with your saved project, where something was removed when it was first created and is no longer present on your system.