I cannot determine an exact pattern. It generally crashes (freezes requiring a system restart) within the first 10 minutes of play, though it has frozen longer into things.

I am happy to supply any files you need to diagnose.

The only interesting thing I have seen when this happens, is that there is more than one java engine running at the time. It seems like a thread doesn't get destroyed when the launcher is closed perhaps? I have noticed that this appears to be happening even more frequently now that the new launcher (1.1.1) is being used...