As I mentioned in another thread, traffic turning on and off, taken on its own, sounds more like DNS Cache Poisoning [webmasterworld.com] than something Google is doing.
Just went back to reread that thread again (what a read) and, at a first look, does sound like it could be a cause. But then I have to go back to drall's numbers which were what got me interested in the throttling topic in the first place. I just can't see/understand how a highjacked cache would produce that almost exact 36K Google referrals per day. And if it were a highjack, wouldn't it also affect referrals from other SEs? Haven't seen any mentions of problems with Bing, maybe just not looked for.