Try a new toner cartridge.
My HL-2040 printer. Has toner in it, it was printing very good. but then the toner light flashs, and then prints, then the toner light flashs and so on. It prints fine but the toner light flashs and stop then flashing. I do not think i need new toner. Maybe the senses have dust of them.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic