Crashing in midst of play

Travelling in the Nether, I got the following:

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007fa3b52f1472, pid=12458, tid=140340024383232
#
# JRE version: 6.0_24-b24
# Java VM: OpenJDK 64-Bit Server VM (20.0-b12 mixed mode linux-amd64 compressed oops)
# Derivative: IcedTea6 1.11.5
# Distribution: Ubuntu 12.04 LTS, package 6b24-1.11.5-0ubuntu1~12.04.1
# Problematic frame:
# J  amx.a(DDD)Lamw;
#
# An error report file with more information is saved as:
# /home/dmh/Games/Minecraft/hs_err_pid12458.log
#
# If you would like to submit a bug report, please include
# instructions how to reproduce the bug and visit:
#   https://bugs.launchpad.net/ubuntu/+source/openjdk-6/
#
/home/dmh/scripts/minecraft: line 4: 12458 Aborted                 (core dumped) java -Xmx1024M -Xms512M -cp minecraft.jar net.minecraft.LauncherFrame

----

When I saved and reloaded, it worked for a while and then crashed again:

INFO: Saving chunks for level 'August'/Overworld
Nov 3, 2012 8:04:18 PM net.minecraft.server.MinecraftServer a
INFO: Saving chunks for level 'August'/Nether
Nov 3, 2012 8:04:18 PM net.minecraft.server.MinecraftServer a
INFO: Saving chunks for level 'August'/The End
/home/dmh/scripts/minecraft: line 4: 17571 Trace/breakpoint trap   (core dumped) java -Xmx1024M -Xms512M -cp minecraft.jar net.minecraft.LauncherFrame