Blaze Spawner In Room Causes Crash

I have a blaze spawner in a Nether Fortress around which I have built a room with hoppers as a floor, connecting to a chest.

The front of the room is glass, with a 1 block hole in front of which I stand and attack the spawned blazes with a sword after about 10 seconds of standing here and attacking blazes, the game just freezes and crashes.