Yep forum here too. I'm starting to think there's an alternate index algo just for the slightly larger sites, we tend to get hit 'off season' - during Panda/Penguin stuff we rise, and then get knocked around in these 'non-mozcast' days.
I dropped from 180k daily to 140k - never recovered. We don't involve ourselves much in SEO, most of what we've done started in the early 2000s and we deliberately ignored SEO pattern building.
Is there a group we can start, where Webmasters of these size/type of sites can communicate info with each other? Posts here and other other SEO sites don't help anyway, it's almost exactly opposite to what we experience.