tedster - 4:18 am on Jun 6, 2012 (gmt 0)
None of the possible explanations I have thought of to date would can months and months of such a controlled pattern. Those who are mostly looking at conversions, can you segment your total search traffic during "on" and "off" periods in two ways: by 1) keyword and 2) by geography.
It seems to me that if this is a "real" pattern (even though rare) then shifts in one of those two mechanisms could well be in use. Of course, you may uncover some other mechanism, but there MUST be some mechanism. Otherwise the controlled or throttled traffic explanation starts to sound almost like a fantasy.
If we can nail down a mechanism, then we might have a shot at understanding when and why a website would trigger this kind of treatment.
It would also help if people share what volumes of conversion we're talking about. At lower levels (single digits over 24 hours, say) it's a lot harder to see statistical significance over short durations - so a many day or week pattern would be more important.