I've been wandering about in the nether trying to find a fortress. I now find that when I try to re-enter particular chunk(s), Minecraft freezes and crashes. Changing the view settings sometimes allows me to move further, sometimes makes no difference. But now merely moving into certain areas of the world makes the game freeze and crash. These are old chunks that I'd moved through before and lit up with torches. Can't tell if my exit portal is in the affected chunk(s). I'm worried it may be.

I haven't encountered a crash problem this severe or repeatable for many months. Now it's suddenly started up, mid-game, in a reasonably fresh world that's just a few weeks old. Previous explorations in the same part of the nether had no problems at all, but now if I enter the wrong area, I'll crash the game. Some directions are still safe, but in other directions, one block too close to I-don't-know-what will cause a crash.

I presume it's something corrupt in a nearby chunk, but that shouldn't crash the game.

Logfile attached.