HP LaserJet 200 - Print wide horizontal lines

I have a HP 200 printer that if it sets unused over night prints completely black pages that lighten over time, until you can print BOL. If the print sets unused fro 10 minutes you get wide horizontal lines. We have tried replacing the toner cartridge several times. No luck.

Can anyone tell me if this is a fuser issue or our we having bad luck with cartridges.

Thanks

Charles