Two Different Sites Disappear Together?

I have two sites that disappear together the same day from top 1000 and then come back together after a week or so to page 1 also on the same day, and it keeps happening, they are not interlinked, they are different niches, and they are both about 3 months old, they are on the same ip but I have 20 other sites on the same ip that don't disappear, so these two must share something in common, I just can't tell what it is since I use the same seo strategies for all my sites. Is there a way to stop this cycle from happening?

I don't think it's a dance since they've been stable and slowly ranking up to page 1 for 2.5 months now and then suddenly this started happening, I also don't think it's a sandbox or manual ban, since they keep reappearing on page 1 and then disappearing again and I haven't been doing any recent seo work for either of them. Did anybody experience this before?

do you have them listed in your webmaster tools? also with them getting into the domain game you will see in your web master tools other sites that they have pulled that they think you are the owner of and they want you to check if its your site or not.

IMO its just the same old google dance.

Sheeeee google knows just about everything.

I said just about.

I don't see them in webmasters tools and I never add my bh domains. I have whois guard installed for them so I don't think they can detect the website owner unless they have a way around that. But then again if they can detect these two sites why wouldn't the rest of the sites go in and out of google serp as well?
The funny thing is that they picked the most valuable website to screw with, so I'm either really unlucky or they are really good at pissing blackhatters off.