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Normally the issue you're describing is one of a few possible things. It could be the imaging kit as you mentioned, the toner or a fuser issue. Possibly also dirty rollers.
If you're using original Lexmark toners and assuming all the inside of the printer doesn't have any leaked toner and the rollers that move the paper through the printer aren't dirty, the first thing to try is a replacement imaging unit to see if that fixes the issue. If that doesn't fix the issue I'd then try replacing the toner. If the toner's at fault, you would normally see some kind of mess inside the printer.
I'm sorry I couldn't give you a more definitive cause for your problem, but it's quite difficult without seeing the printer in person. If you're using cartridges that you've purchased from us, we may be able to help further if you give us a call on 0345 365 3605.