tedster - 5:14 am on Dec 15, 2012 (gmt 0)
OK - so far our analysis of "zombie traffic" seems to have come up with one pretty solid explanation for some cases: mobile traffic. I can see how that might create the on-again off-again phenomenon - if Google were testing the site in mobile search results.
How might a stable total level of traffic ("throttling") also be a result of mobile traffic? That part I cannot even theorize about.