The problem with instabilities is that they appear randomly. That's why longer periods of stress testing are more accurate at finding problems. I recommend testing for at least 8 hours with prime95 or Orthos before declaring a machine stable. All this shouldn't be an issue with stock settings though. I would return to the optimized defaults and make sure you are stable at stock settings before attempting any tweaking. You should be ok with the AUTO values that it sets, that's all I use for stock speed.