hp laserjet 1020 printer problem

I have a problem that is cropping up on several hp 1020 laserjet printers and I wonder if anyone is having the same problem. After printer has been used for awhile, the printed pages have some shadow print from lines of print on the same page. I have changed the toner cartridge and the problem goes away—for awhile. I tried cleaning contacts etc but that doesn’t seem to help. The toner carts are not cheap and I know they are not even near empty when I have to change them but that seems to be the only way to clear up the “shadowing”. Help!