PHASER 7500 repeating marks / smears on border

Hello.

I own a Phaser 7500 and have troubles with repeating marks or smears on the lower side of the sheet - the one closer to the printer's screen. The marks are black and 8,2mm apart (see picture).

The troubleshooting page states that is a bad fuser but I checked it out and it's perfectly working. Also I took out the accumulator belt and found these identical toner marks on it - so this happens before the fuser anyway.

I checked the drums and they seem ok, I also cleaned everything.

The interesting part: I did the "Cleaning for repeating artifacts" on the printer menu. It prints 5 pages and the first one has strong marks, the second one a little less, the third one even less and the fifth one has no marks on it. But whenever I repeat this process the marks come back on the first page.

UPDATE: I did the "drums test" with 4 CMYK pages and they all print fine, but they all have the same black marks on the bottom. So it is not the bad drums.

Any ideas?