• Toner cartridge is low on toner. Try gently rocking the cartridge to redistribute the toner. This will buy you 50-100 pages. Get a new cartridge as soon as possible.
• Dirty or misaligned contacts on the high voltage power supply.
• On the HP II/III printers it can be a loose connector on DC controller see tweaking for instructions on how to repair it.
• On the HP 4/4+/5 printers there is a potentiometer adjustment on the left side of the printer. You have to remove the left panel and you'll see it. It's located at about the exact center of the printer.