bluntforce - 8:41 pm on Sep 18, 2012 (gmt 0)
I've seen both what appeared to be throttling and the "bunched" signup issues. I believe apparent throttling is just a byproduct of an established website having stable positions for some given key phrases. When I look at Google, Bing, Yahoo and AOL, Google will produce 6-7 times the visitors as Bing and Yahoo, AOL produces 1/3rd as many as Bing/Yahoo. Daily/hourly fluctuations don't mean much but I'll look a little closer if it appears there is a trend. AOL only having significance because of the relationship with Google. I'll see overall search volume drop at various times, but as long as the percentages between the search engines remain consistent it's a user thing, not search related. Average search query on Google is now something over 4 words, I've expanded 2 word phrases making them more diverse and inclusive of transactional type terms.
I've never been able to determine a relationship between "bunched" purchases although I use Piwik, GA, a log analyzer and a database program for in-depth analysis.