You're just describing normal SERPS variations lorien. The safety net is applied on big money terms. Google has in the past shown an ability to treat competitive phrases differently.
No, I'm not. The drop is across the board. Hundreds of pages dropped between 4 and 7 spots. It's not a normal variation at all, certainly nothing with a past history. It could be that aged links are being discounted - that they now have a lifespan, which would be an interesting concept.
The competing sites that have cropped up to the top are ones I recognize from 3-4 years ago, mainly very thin affiliate sites and thin content sites. The affiliate sites that have cropped up are ones that hide the affiliate code in a form tag, which google does not follow; as far as I know.