Something Google does every so often looks like traffic throttling or rationing.
I agree with the "looks like" part, but I doubt there is any actual throttling or rationing going on, only the appearance of such. Google has rolled out a lot of changes this summer, and something this complex is not going to be perfect. I do believe this is a perfect storm of many factors, the economy being a large part. The only time I ever see wild fluctuations is when a new algo is brought online. It usually clears up after a few days or weeks. In the mean time, the only thing those affected can do is wait, re-examine their sites and hope it all settles out soon.