Hi Dave,
I never use the "post reply" button on the thread page - I always use the big green "reply" button underneath the last post or alternatively if I want to quote a previous post, I use the small green "reply" button underneath that particular post.  I then, of course, press "post reply" when I've written my post on that screen and I never have any problems!  Not sure if I've made myself very clear but hope you understand what I mean!!
Cheers,
Bob