My LaserJet 4M has suddenly developed a serious problem -- just as I need it
to get out some important documents.

It's printing black lines across the width of the paper, about 1/8" to 1/4"
wide. They occur randomly (two to three per page) and look as if someone has
run a Magic Marker across the document.

In one case where the line ran through a graphic I use on my letterhead, the
line appeared to be "behind" the graphic -- that is, the white spaces in it
were reproduced with little degradation.

This does not appear to be a toner-cartridge problem. It seems more likely
to be intermittent laser output. (When the laser is off, the drum retains
its charge and picks up toner that's then transferred to the paper.)

I need this unit ASAP, and am willing to rip into it in the hopes that the
problem is a loose cable or failing solder joint. (I can download the
service manual for $7, if need be.)

Any ideas, anyone? Thanks in advance.