Mine did the same thing. After troubleshooting the most common problems, I started to suspect it was the power supply board. A little more research indicated a lot of 3030's begin to exhibit this behavior after 5-6 years. Luckily I ran into the following posted on a message board. I tested the capacitors referenced and found the 47uF/35V had all failed. After replacement, the printer has "as new" image quality. Hope this helps someone.
Forget buying a new toner cartridge and don't bother re-installing anything - This is a power supply problem - The electrolytic capacitors fade away after a few years of power on and the high voltage to the printing engine drops too low.I tried a new toner cartridge - no help - re-installed drivers - no help.If you can handle a soldering iron or know a friend who can, replace the following capacitors on the power supply board on the LHS (power cord side). Make sure you use low impedance 105 degree power supply types. Warning - high voltage !!! power cord out and wait 30 minutes before touching anything. C374 341 503 529 (47uF/35V), C526 (1uf/50V), C506 (0.47uF/35V), C515 (100uf/50V). All the others tested fine in mine. Also make sure you install them the correct way around! Now I have "black as black can be" prints just like new and I set print density back to 2 not 5 to save toner. enjoy ;-) - Ray P