"The printer is fine."
How do you know this?
Do these random lines only occur when printing PDFs from AutoCAD DWGs? (Do you make hardcopy print of PDFs from any other applications?)
Still, I would recommend running it through any maintenance procedure it offers, if for no other reason than to rule it out.
Is this an ink-jet or a laser/toner type printer?
Do those "random" lines correspond to any geometry in the DWG? (Maybe lines or edges that are supposed to be hidden? Of course, it seems rather unlikely that such an object would be invisible on the PDF yet be visible when printed to a hardcopy printer.)
Can you try changing the PDF quality (resolution) settings to see if this makes any difference?
Can you post a copy of a PDF that does this for you, so maybe we can try to duplicate the problem with our printers, or perhaps see something in the PDF that explains it?