Lexmark t520 suddenly prints poor quality, i.e.

Start trying a new tonercartridge.
Gerrit Jan

this sounds like you have a fuser problem look at the fuser and see if it has toner build up on the rollers ghost printing can be caused by the toner cartridge or the fuser. this would indicate that your fuser isnt maintaining enough heat for complete fusing. is you take the rear covers off and run your print job. look to the far left side of the fuser and you will see the heat lamp lit up. this light should cycle on every 1 1/2 pages or so. any longer than that and it wont keep up with the print job