tedster - 9:58 pm on Jan 8, 2011 (gmt 0)
Hello fifi, and welcome to the forums.
This kind of phenomenon is a very puzzling thing, isn't it? A good number of reports have been made here for over two years, and we still don't have a definitive answer. See Google Traffic Throttling - revisited [webmasterworld.com] for a look at the ideas we've been floating.
Google shut down a number of our largest sites entirely, and for no reason that I can figure out.
Good luck with figuring that out. You may find food for thought in the Hot Topics and FAQs area [webmasterworld.com], which is always pinned to the top of this forum's index page.