Thanks Foamcow. No I'm not opening the document over a network connection - it's on my internal HD. I do copy/paste into a new document and rename/save. This doc will eventually inherit the same problem.
I'd like to find the cause of the problem though.
It has been suggested that it is caused by corrupted font&/or previews. Previews can be rebuilt as follows:-
Bring up the "open" dialog box
Select your file (don't do anything yet...)
Hold down the "command" key
Click the "open" button (or double click on the file name)
You'll be presented with a new dialog asking you what you want to do:
keep the existing previews, build new ones, or make them into gray boxes.
I,ve tried this but Quark bypasses the dialogue box so I assume my previews are ok.