bluntforce - 7:00 am on Apr 8, 2012 (gmt 0)
I don't know about zombie traffic, but I've seen traffic that converts flow along nicely for part of a day, then it shuts off, only to resume some number of hours later. It used to be weekends were totally dead, but that changed over a year ago.
From what I've seen, it's keyword related, not page related as a given page will apparently retain it's position in the serps for one keyword but will drop significantly for another keyword.
With that said, a shorter term way to deal with it is to promote alternate keywords/phrases. If a "main" keyword/phrase is reduced, the alternates keep things going.
My personal impression is it's taxonomy/architecture based. The algorithm/classifier can't quite figure out where a page should fit given the information it has.
I've recently gone the "noindex" route with some pages that received authoritative inbound links in the last year or so, but they aren't pages that generate income or thematically support income producing pages. I'll give it a month or two to see how that goes.
I know traffic shaping/zombie traffic is real because I've seen it, but I think it's more related to individual site clarity/focus rather than a search engine agenda. Like an electrical breaker, power goes through until it trips, then nothing until it's reset.