I could be wrong, but I think this issue may relate to the web server. I
realise now that when we had major problems with this issue it was only
apparent on one site where we hosted on a different sever, the ten or so other
contribute sites we managed were all on the same server platform.

So it might be worth uploading the site to a another server to check to see if
its an issue caused my the web server somehow.

Hope this helps!