Dell 3110cn horizontal black lines - help!

Hi,

My charity's printer is barely used, but it's printing horizontal lines on every page. For example, if I print black, it produces 3mm horizontal lines every 29mm. They are all the original toner cartridges. I've printed a check page, so I can see the distance of each line as 29mm. It says on the final printed check page that 28.8mm is the "printer cartridge". But I also note that all colours are printing the same line, at the same distance apart. How can they all be suffering from the same problem?

Is there any way to clean this? It would cost our charity a lot to change all the cartridges.

Thanks for any help,

Oliver