I have been monitoring this since your last post. It's still happening on some sites but not others. I cannot determine any common thread at this point. I cannot be the server or none would work correctly - possibly a conflict with another component/module/plugin, but, as I said, I have been unable to determine any commonality between misbehaving sites.

So I disagree with the conclusion in your last post. It's definitely something on the site, not the server. If it was the server, I'd have all of them doing this.