There is no need to simply replace all toner cartridges without understanding why, you state you have changed them and the post above simply says replace them, the cost of this is crazy - what proof is there that a cartridge is faulty ?
The post given may be for a different scenario so simply referencing another solution without qualification is not good practice, effectively your throwing parts at it in a vain hope that it will fix itself but at a high cost.
These printers have on board diagnostics, you can print a PQ troubleshooting page first to see if the defect is in any colour and also see if you can find a repetition, you can also use CARTRIDGE CHECK DISABLE to remove one or more cartridges, this will allow you to trouble shoot individual cartridges and sometimes see any defects clearer.
A visual inspection of the fuser and belt are worthwhile if you find all colours produce errors, in those cases you can also use the STOP TEST in the diagnostics menu, get the printer to stop during print, open it and see if the defect is on the media etc. If you find no error on the media and the belt is clean then you may have a defect on the fuser but tha main this is to walk the fault through, running these tests will only take 10 minutes but is a lot simpler than almost £900 on a set of cartridges.
Replacing all toners without good reason is simply throwing money away and will do little to narrow the issue down, the proof is you say you have already changed them so the issue needs some tests to confirm where the root cause lies.