Intermittent failure. Intermittent failure:
Carmen Crincoli of Microsoft points out a frequent cause of
intermittent failures which cannot be produced on demand: heat buildup.
Things can change if your cooling vents or fan get too much dust. It's
also a good essay on how people “on the other end of the phone” try to
discover the difference-that-makes-a-difference to likewise achieve a
failure. [JD on MX]