But IMHO "on / off" traffic patterns and Zombie Traffic are one in the same. It may not have anything to do with algos. But I'm not ruling them out.
I believe that much of the problem lies with the general dilution of the entire landscape in any niche. Years ago, one or two sites could stand out against a basically blank background landscape of sites. Now with more and more scrapers, wannabes and photo spam sites, it's harder to stand out. In our case, this lowers the number of potential customers who need our products because if they only needed a small portion of info, they are finding it on the scraper sites who offer it for free today. They offer it for free so visitors will come to their sites and hopefully leave via one of their many adsense ads.
The problem with this theory is that many tangible product sites (like ohno and many others) are seeing the same patterns we see. Tangible product sites should not be affected by scrapers. After all, they can't scrape the physical products that the customer needs, so we're back to suspecting the algo and database structure and refresh methods.
This also does not explain the sudden ON / OFF conversion periods, unless the scrapers (or poor quality diversionary sites) suddenly had disappeared. However, this could be explained by the new, improved, faster everflux of Google's serp data.
The how and why's escape me though.