We had a similar (though not the exact) problem and it turned out to be related to the combination of the browser itself, and the data inside a TEXTAREA field (basically an odd browser bug). In fact, sometimes the SUBMIT button would not even work for some odd reason.
So the problem could be on the client, rather than the server (though it does sound like you've narrowed it down to the server).
Have you tried analyzing the HTTP conversation? Or using a non-brower (command-line utility) to do the POST-ing repeatedly?