I think this might be the same issue I am having, but I have many layered materials with AO, not just one. I can render for 30-60 minutes before it crashes, sometimes it even finishes rendering with no crash. Updated my daily build, restarted my PC, merged into a new scene, but it still crashes. I am currently half way through a render, so I will see if this one crashes too.
Edit: The render didn't crash, but the next one did.