Sounds like a total hunk of junk, dunno where you got it from but I'd take it back and throw it through their window. :D
I'm impressed how long you have kept those poor old girls running, which one is this the low or high mileage one ?
The thing about that carousel engine is not a lot of it is specific to one colour, so if you are only getting bands in one colour then you need to take out that colour and then open the side access up for the drum and have a good look.
Bear in mind though that contamination of all the colours can look an awful lot like just black contamination unless you inspect it on the drum carefully.
If you print 40 blank pages do the lines fade ? That could be as simple as toner spill inside the engine getting on the transfer roller.
If they fade and then reappear once you start printing again then it is probably the charge roller.
Otherwise I am afraid it might be the drum despite the life showing :/
Best to go through the very good troubleshooting section in the user guide (still available as a download from HP)
http://h20000.www2.hp.com/bizsupport/TechSupport/CoreRedirect.jsp?redirectRe...
The other thing to check is that if you haven't emptied it recently that the waste toner tray is empty But I don't think it causes that specific problem.
Instructions for that are here anyway
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=bpl07...