Reno - 12:25 am on Sep 23, 2010 (gmt 0)
It seems to me that in order to throttle traffic the site must be absented from the SERPS
Not necessarily absent ~ just being dropped a lot lower would do the trick ~ you may recall the discussion here about the percentage drop from #1 to #2, then to #3, etc ... if you go off the first page, you are in the hinderlands. So suppose all that Google did was add a +11 to your domain during your "throttle times"? You're still there, but you're essentially invisible. Not saying that's the case, but it's one scenario. The test is to methodically check position for a small handful of keyword phrases at regular intervals throughout the day, and do that for several days running ~ if nothing changes, then it shoots down my thought; if however the results are quite inconsistent, it may explain a lot.