I've noticed the same thing as well.
It seems that if you are typing a response to a thread, and it takes a few minutes to type and you then hit post, sometimes it times out.
What I have to do is type out the response, then copy the response then I hit the back button on my browser, then hit forward on my browser, then paste the response and immediately hit the "POST" button, then 9 times out of 10 it posts the response straight away.