Crashes

My minecraft is crashing! It did not have a problem in 1.2.5, but as soon as I updated to 1.3.1! It is unmoded and the java is up to date. I download the snapshot every week. My game WAS MODDED, so it has some useless stuff in .minecraft folder, but it started crashing before I had a modded game. It does not crash on launch but often it freezes and/or crashes. It is more often on new world save with more then 1 player and only freezes on multiplayer. pleaase help me!