I recently moved my 13 year old site from http to https. Traffic wise It's had it's ups and downs throughout the years mainly do to Penguin.
A couple of months before the switch I had a page (with lots of good related info to the site topic) which dropped from page 1 #4 spot to page 3-4. This page has been cited and linked to numerous times by major newspapers but it's also had it's share of scrapers and bad links (applied by others). The page had been on page 1 for probably over 6 years.
When I switched to https with 2 days it popped back to it's spot on page 1. I though great it must have been the extra SEO love from now being https. Well within a week and half it has now moved back to page 3-4 (while the rest of the site pages maintained it's positions).
My thoughts on this is while the page is still seen by the main Google Algorithm as perfectly fine and ranked it well, it's being dropped in the ranks by something like Penguin which isn't part of the main algorithm and is a week plus behind in applying it's ranking info.
I have since then applied over 10 more domains to my disavow file that point to this page to see if it makes a difference within hopefully 2 weeks. I will report back if it does or not.