Reversed plot problem

This is a new one for me. We have one user here that prints a lot of
drawings that have fairly large jpeg images in them. But intermittently,
with nothing changed at all in AutoCAD or anywhere else, the plotter will
start plotting with black and white reversed, while the color images are
just fine. By reversed, I mean all the linework and text that should be
black will be white, while the complete rest of the paper background plots
in completely black!! Except for the normal color jpeg image portions, it's
like looking at a negative of a complete drawing!! Plot previews are fine,
(normal).

This is in LDT/Map 2005, Windows XP, and using a DesignJet 1050c, with
everything current. It only does it if there are images in the drawing, and
only intermittently. I can find no options for reverse printing, etc.,
anywhere, and just wondered if this has happened to anyone else and if
anyone's found how to prevent this. Recover and audit make no difference.

Thanks for any suggestions - John