I have had this problem on two worlds, and it has prevented me from accessing them. I have found dungeons with mob spawners on them, one world has skeletons and the other zombies; I am using them in conjunction with a redstone powered hopper/dropper system.

Initially the world loads without a problem, but as I build more thing in the world the game starts to bog down when I am near the spawner, and suddenly it starts closing the game, but it allows the world to be loaded once more, until the final crash where the world simply stops loading; the terrain generation dialog ends and I get a flat color on the screen, just above the GUI, and the game closes.

I have tried changing the difficulty to peaceful to eliminate mobs spawning, and also changing modes to creative, both with and without mobs, but the world just does not load. This has happened on two worlds, with two different seeds, but has not happened on worlds where I have not found spawners; I have exported the world file to other machines, with higher and lower tech specs, and the result has been the same in all three.