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Around 18th through 20th, google would give our site a 50 % recovery for 3 days.
Everything goes back to "normal" after 3 days (normal meaning loss of traffic).
Is this the so called yo yo effect?
Using the yo-yo mechanism to gradually release a url from a penalty sounds like a step Google might take. They could gather some good data about the url's performance at a higher position that might be useful for them.
I have the same rankings I've had for ages for only 3-4 days at a time then drop out for a week or more only to recover for 3-4 days again.
It's an old site that's been around since 2001. Haven't made any changes that would cause this to happen.