Thanks for the replies. I just want to clarify that this software has worked perfectly well on another site for nearly a year, so there is something up in the second site's infrastructure that's causing this. I should just put no-caching into the default master and that would probably solve the problem, but I'm also interested in learning something here.
One important fact I forgot to mention is that the caching is intermittent and not yet reproducible.
Thanks again.
