The other day I was playing my creative world as normal. Then, i closed and updated to 13w19a. When I entered the world again, I was lagging at 5-0 FPS. When I finally got out of the lag zone, I was on normal 80fps again. What happens, is that when I cross X:226 or something like that, my game goes from 80 to 0 FPS and then crashes. Please, please help me, I can't lose this world, I've been building it for a year now.

(Btw, could it be that my chunk got corrupted? Because the rest of the world works fine, it's just that one or two chunks that make the game crash)