I had a similar problem with my HP 4. The first thing I did was clean the dust brush in the output path. The next step was to replace the toner cartridge. Finally, I took a vacuum cleaner to the thing using the skinny head and picked up every bit of crud and crap I could get out of the whole thing. That was six months ago and I haven't had a problem since.

I know how frustrating these things can get--and how hideous the printing can look until you get it right. It seems the original problem was the toner cart. That made such a mess that the rest was necessary. If you haven't disposed of your drum assembly, it might be salvageable with a good cleaning.