oskar - A couple of quick things to try... is your printer hooked up to the electric through a surge protector, or anything that might limit the power it gets? Also, try recalibrating if you haven't already, refer to the manual or google if you don't know how.

Failing that, here's an email I got off my friend (who has seen your image posted above), he is a former printer refurbisher so he's not talking out of his ass...

Primary reason for dropouts is low toner, give the cartridge a good rattle and put it back in. Any better?
Yes? Ace! But you'll need to buy more toner soon.
No? Right...
Does the black on the image scratch off easily?
Yes? Fuser is fucked, throw it away and buy a new one.
No? Right...
Can you see a spongy black a4 width roller anywhere inside the printer if yes, you should be able to clean it with a soft dust free cloth. A bit of alcohol on it will help DO NOT USE WATER.
Did cleaning this roller help? (Might make it worse for the first print but 2nd should be clear)
Yes! Woot! You might have to do this every so often, the transfer roller is electrostatically charged so toner sticks to it like flys on sh....
No, Right...
Now threaten it with a hammer...
If none of this helps, then it's probably a fault with the machines High Voltage power supply unit and it's basically now a very large heavy doorstop. Put it out of it's misery.
DO NOT PURCHASE AN HP MACHINE TO REPLACE IT UNDER ANY CIRCUMSTANCES.


Kindest


Alex