M281fdw printing lots of black spots on back of page

[Bettercallsol]

Printer is producing pages with blotchy spots on the back of the pages. I've checked the toner cartridges, run a number of cleaning pages through and also check the back roller which has a slight amount of toner in it but the pattern is consistent on each page of spots. The pattern of spots repeats every 5cm roughly, so thinking it can't be the cartridges themselves. Any idea would be greatly appreciated!

[Repairatrooper]

according to the repetitive defect chart, nothing correlates to 5cm, the closest is 51.5mm for the secondary transfer roller (open back door and look for spongy roller on the door) and 56.5mm for a fuser problem.   (fuser is very hard to see on this engine design and you pretty much need to take the printer apart and remove the fuser to see anything)

[Bettercallsol]

Thanks very much for your response. I've measured and I believe 5.15CM (51.5MM) is about correct. I've tried removing the transfer roller and cleaning it down with a cloth but still after replacement spots are all over the back of the page. Could it potentially be that the Black toner cartridge (only Black spots) is dropping excess toner on the belt and the back of the page picks this up then in turn getting it rolled down the page by the transfer roller? I would say spot patter gets slightly fainter down the page. 

Many thanks 

[Bettercallsol · Accepted solution]

I suspected that the cartridge was of poor quality and spillt toner each time it opened, on the transfer belt(I think it's called that) so I replaced it and it's now no longer having this issue. Just an FYI. Thanks for the sheet of measurements @repairatrooper!