Every once in a while the client will just stop syncing. The easiest solution is to kill the "chain" directory in the data directory and re-sync. I realize this is frustrating, we're working out little edge cases like this as fast as we can get to them.
@charles No, the plan is to have checkpointed releases as soon as we have a way to pre-package the chain DB, at that point we can also make "official" checkpointed torrents but so can anybody else.