I notice my EPC goes down when my traffic goes up, and most of my sites are seasonal so it's pretty easy to see when my traffic spikes. I don't believe in a throttle, but I do think that some advertisers might see a sudden spike in clicks (as my traffic goes up) and may stop advertising on my site. It's backward, but a lot of advertisers aren't educated on such things.
There's also availability, and third party networks, and all the other things that go in to the calculation at the exact moment of a click.
But I'm pretty sure there's no throttle. My EPCs for my main sites this year are close to twice what they were last year, and the "drop" when my traffic spiked was far less than usual.