Strange grey squares when viewing postscript file

[ajheaps]

Hi,
I've recently installed OpenSuse 11.1 64 bit on a new computer and I'm using ImageMagick 6.4.3 that came with the operating system. When I try:
convert -rotate -90 idl.ps idl.png and then display idl.png
or
display -rotate -90 idl.ps

I see strange grey squares instead of the white background to the plot that I was expecting from the postscript file.

The snapshot of the screen (test.png), original postscript file (idl.ps) and output PNG file (idl.png) can be downloaded from:
http://ncas-climate.nerc.ac.uk/ajh/imagemagick

If I look at the output file (idl.png) with display I see the same strange grey sqares as in test.png. When viewed with xv I see a white background as expected.

Does anyone have an idea why I'm seeing these strange grey squares instread of the expected white background?