PHP 5.1.4 not populating $_POST array on IIS6 reliably

Hi all, We've got an odd one here. Windows 2003 SP1, IIS6 running with PHP 5.1.4 as an ISAPI module. Form submission method is POST, and the data is coming from a form inside outlook 2003 (although 2000 and 2002 versions exhibit the same issue). The issue is this: Around 10% to 15% of the time PHP will not populate the $_POST variable when the form is submitted from Outlook. Things we have figured out: 1) PHP 5.1.4 is the most reliable (5.0.5 is terrible on this, 5.1.2 is somewhere in between). 2) Outlook is always POSTing the data to the web server (I used a packet sniffer to make sure of this). 3) It *always* works if we submit to an ASP handler page on the same IIS server. 4) We've ruled out any network devices (such as firewalls) by running a test webserver inside our firewall (again, 2003 SP1 with IIS6 and PHP 5.1.4) 5) The $_POST variable is always populated if submitting from a web browser, or a web based email client. 6) The failure sequence is random - sometimes you can submit 15 times in a row correctly, sometime it will fail on the first attempt. 7) We have made a test form HTML (for sending in an email) and a test handler (which does print_r($_POST); and nothing else) - we can see failures even with that - files can be sent on request. I've been going mad over this ... any help would be appreciated. Thanks Jason