diberry - 4:42 pm on Apr 11, 2013 (gmt 0)
I actually have noticed something fairly similar to what you're talking about. I leave quality comments on blogs in my niche, and they always bring at least a few people over to check out my site. But once in a while, I'll have something really impressive to say and that link will bring a lot more visitors than usual. The day after that, it's like Google compensates by dropping traffic levels.
I think it might be a side effect of Google re-evaluating our link profiles. Why they would do it every time, I don't know. But I think it's a pattern that could easily go unnoticed by webmasters because it's fairly subtle.