Game Freezes on "Loading Terrain", Multiplayer Servers crash at random times (mostly command use)

In the new Minecraft launcher (the old version works), when I attempt to create a new single player world or join an existing one, the game freezes on "Building Terrain" or "0%". I am able to join multiplayer servers, however at random moments, especially when using commands, the game will crash completely. I have tried reinstalling completely, removing all mods and clean installing. None of these worked, however the old Minecraft launcher allowed me to use commands, create worlds, etc. Also, on a less urgent note, I appear to not be able to change my skin in either launcher, as when I change it in the launcher or on minecraft.net, it stays as the default.