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And I found out that they were all linking to another website of mine.
The thing that confuses me the most is how old keywords are still getting flawless #1 rankings while the new content pretty much gets ignored.
Have you checked if your newer posts are taken/copied/stolen by other sites and republished with or without backlinks to your site?
Google recently said that they have whitelists so that specific algorithms don't apply to specific sites. For example, if they determine that using purple links is correlated with spammy sites, they might put purple links in the algorithm as a negative factor. But they know of some big, reputable sites that use purple links. So they put those sites in the white list for purple links. Those specific sites don't get hit when this algorithm rolls out.