Autocad 2002 full plot preview different than plot

I am using ADT 3.3 (Autocad 2002) with a Xerox 510 wide format
plotter. The Xerox plot driver has a great feature - lines merge -
that allows solid lines and gradients to "merge," yielding a plot that
has solid lines visible "on top" of a shaded background. It's a great
feature, and I take full advantage of it. The problem I am
encountering is that the full plot preview has begun displaying the
plot innaccurately. All of the fundamental plot parameters are still
correct in the preview, but the lines merge feature is no longer
"working" in the plot display. The drawing is still plotting
correctly (thankfully) - with shaded regions under the other lines -
but the full plot preview shows the shaded region obscuring most of
the other lines.
As for the drawing itself, the shading occurs in a series of
elevations in another drawing, which was then inserted as an x-ref.
Dimension lines were then superimposed on the x-ref in model space,
and seven paper space sheets with numerous viewports were composed for
plotting purposes. It is a complex drawing, and in hindsight, it
would have been better if the x-ref information had been in model
space, rather than inserted as an x-ref. I know Autocad can struggle
with sort order in display and plotting - and particularly with x-refs
- but ever since we got the Xerox 510 (and installed it's drivers),
everything has worked beautifully.
I have installed the most current Xerox Autocad 2002 HDI driver
(11.0.2 - 7.1.15.30 [U5-1]), and the most current HPGL/2 driver
(12.7.3.2910 / 12.7.3.3898 / 12.7.3.2035), and they are working
properly, with no apparent conflicts. Any suggestions out there as to
how I can get the plot preview to be accurate once again?

ps - I removed all previous instances of the Xerox drivers as they
suggest, and have installed and uninstalled the new drivers three
times, with the same result each time. I'm using Windows XP Pro in a
network environment.