Header function on IIS vs. Apache

I am tweaking a PHP web app developed on Apache/Linux/PHP3 to work on IIS/Win2k/PHP4. I have encountered an unusual situation with the header() function in PHP. When a user submits a form, depending on the results they are redirected to a certain URL using the Header function (usually something like /folder/TheFile.php3?this=that). This works on the Apache/Linux/PHP3 setup but not on the latter - it generates one of those unhelpful 502 CGI Errors (The specified CGI application misbehaved by not returning a complete set of HTTP headers. The headers it did return are:). In some cases just refreshing the page will cause it to load properly; in other cases, not. I know a code sample should follow but the application is just too complex (for me) to summarize. However, if anyone has any pointers (something in php.ini?) I would appreciate any clues. Thanks! Emily Witcher