Crashes when im on a server

Im on a server called sg-mc.org. It's a very good server. Ever since Minecraft version 1.5.2 was released, Minecraft would crash whenever I would enter a sub-server (e.g. parkour, survival, etc.). I was wondering what this was and if it can be fixed. I'm no computer whiz, so If you can put it in simpler terms so I can understand? Thanks! Im a Mac OS user, and I do use the Minecraft launcher