Strange. So if you reload that 20 times in a row, it fails some of those times?
Yes. For example, on the quiz I was trying to check just before I posted about the problem, I saved it, then clicked on quiz preview screen - nothing. Went back to home page, back into edit, saved again, tried preview again - still nothing. Tried again half an hour later and it came up straight away, and that's happened on the last two or three quizzes I've written. Each time I've waited three or four minutes before giving up and going back to either edit or the home page. Whether the preview is just very busy at certain times with a lot of people authoring and checking I don't know. It's not just one category that is the problem, it's happened in different categories/sub-categories.