IIS PHP5 and ISA...Not FUN!

Ok, here's my setup:

Server 1 - Domain Controller - (SBS 2003 with ISA 2004)

Server 2 - Web Server 2003 with IIS6 and PHP 5.0.2, MySQL 5.0.22

Web requests on Port 80 are routed by hostname from Server 1 to Server 2 by ISA.

I think this is a PHP configuration problem, hence why I posted it here. What is happening, is when I have a form like....

***sample code***

<?
if ($action=="edit"){

(stuff I want to do here)

Header("location:listmain.php");

}
?>
<form action="editmain.php?action=edit" method="post" enctype="multipart/form-data" name="form1" onsubmit="javascript:return chk()">

<input type="submit" name="Submit" value="Submit" >

</form>

**end of sample***

Typically at large apache hosts, this same code works fine. However, in my server environment I click submit and I typically get "Page cannot be displayed" in my browser with the same URL in my address bar that I called from the form action. Then if I hit F5, I get the main menu again, without the URL changing. I think something is caching my pages, but I can't find it. PLEASE HELP ME! I have inquired on a few forums without luck and this one is driving me nuts! I know something is caching, because when I make a code change, then it take 20 minutes or so for me to be able to view it. I don't have much experience with the php.ini so there could be something there that is default that is messing me up.

Any help would be greatly appreciated. Thanks in advance!