strange ADS in Chrome, already reinstalled system, can't find it, Adware??

Hi guys,
I’m from Germany but mainly surfing on American pages. So when I started noticing weird, reloading German ads on the normal sites I frequent, I started searching…
And NOTHING has found anything so far.

I’m using Chrome (newest version) on a Windows 7 Prof. system.

The ads load instead of webpages I clicked on through my bookmarks, or they load in the page I’m reading and then go on reloading.

They’re small rectangles with a black triangle that says “Vergrößern” (=enlarge). Usually they’re for Skycheck or poker or some betting site. (See screenshots!)

The site I get this most often on is Fitperez.com by Perez Hilton. The screenshots are from that website.

Avast Full system scans & boot - up scans, ESET online scanner, Kaspersky Antivirus Removal Tool, Malwarebytes, Windows Defender… found nothing.

So I reinstalled the whole system. Formatted all my disks.
And still, 3 weeks later, they’re still there.

It’s driving me nuts, I don’t have any clue what else to do. It this just some funny cookie, or Adware, or what??!!

Cheers for any help whatsoever,
Inga