Ghostscript fonts missing

[Milo Thurston <mith@ceh.ac.uk>]

Hello,
I'm running Debian 3.0, and have run into a problem when viewing
ps files with gv. This is typical:

----------------------------------------------------------------
Error: /invalidfont in findfontGNU Ghostscript 6.51: 
Unrecoverable error, exit code 1

Operand stack:
elvetica   Font   Helvetica   327098   Helvetica   --nostringval--   
Courier   NimbusMonL-Regu   (NimbusMonL-Regu)   NimbusMonL-Regu   
(NimbusMonL-Regu)   NimbusMonL-Regu
xecution stack:
%interp_exit   .runexec2   --nostringval--   --nostringval--   
--nostringval--   2   %stopped_push   --nostringval--   
--nostringval--   --nostringval--   false   1   %stopped_push   
1   3   %oparray_pop   1   3   %oparray_pop   .runexec2   
--nostringval--   --nostringval--   --nostringval--   2   %s
---------------------------------------------------------------

The gsfonts package is installed  but I cannot read most postscript 
documents.  Those that I have created with latex & dvips are
readable, though.
I also tried editing /etc/X11/XF86config-4 according to an
archived post describing "missing" fonts after upgrades,
but that did not work either.
Suggestions would be most welcome.
Thanks,
Milo.

Milo Thurston, CEH Oxford, Mansfield Road, Oxford, OX1 3SR.