Your computer might be taking too long to preload textures. If you have the "Preload skins" option checked in the "Video Settings", uncheck it.
If it still happens, then uncheck the other preload option and see if it happens. If it stops, then set the "Maximum Texture Size" to one notch lower and try preloading the textures into system RAM alone.
Things like this are normally caused by the computer not having enough available resources when the game starts and Windows has to swap out a bunch of things to make room for the game. The second you try, everything is still loaded in memory, so the game does not timeout.