I'm seeing some time of day related rankings changes this week. Today for example from midnight to noon traffic was roughly 40% lower than the average and at exactly noon it returned to within 1% of average.
Yesterday it seemed to work in 4 hour bursts. Between 30% and 50% less traffic 4 hours at a time followed by near normal four hours at a time.
I can't attribute this to anything I've done, the site is 6 years old and unchanged in quite a long time. Traffic sample size is typicaly 14,000 visitors a day with 31,000 pageviews so I don't see it being a small sampling issue either.
This is happening with Google only.