Something wicked was screwing up Ubuntu updates recently. Almost made me go back to apt. Just make sure you update your catalogue bwfore downloading and installing new udates. I know with apt this is a no-brainer, but I always assumed that update manager handled that automatically. First one update crashed on restart, the threw a bunch of errors. Then, updates would say there was a bad connection. Finally I had the sense to click check, and found more updates. That did the trick.