The problem is now solved. Not memory or anything like that, but a corrupted font. Our supplier, Quad Logic Systems narrowed it down to a corrupted font (Lubalin Graph in the OS (System Folder/Fonts folder). As we generally only use this font with VW, the problem showed up with VW printing to a postscript printer. Oddly enlugh, this was not a problem when running under OS 9, so - either the font was corrupted subsequently (by X?), or OS X does not like old fonts!
If anyone else has a problem with corrupted fonts, it is important to dump the font entirely, and disable it in the Font Book, before restarting. To make sure it is dumped, open a VW document that uses the font and see if the font substitution dialog appears. If all is OK, reinstall the font from CD or floppy.