.Net 4.0 WebForms App Using Routing?

I have a completed app running in a QA environment. Everything works fine under most circumstances.If you hit a plain URL (no identifying information in the URL), you see an intro page with a button (generated by an asp LinkButton control) that posts back and directs you to another page. The markup looks the same when it fails and when it doesn't.

When such a URL is followed from, e.g., Word and the default browser is IE, the intro page loads fine, but clicking the button causes an error. When not debugging, this behavior occurs every time. While debugging, the error occurs only ~ 1 in 10 times (closing the browser instance and starting over every time).When the error occurs, the intro page Page_Load fires and IsPostBack is false. Somehow, instead of a post, a get is being issued.

When I run fiddler to try to analyze the actual calls (can't use firebug because it never happens using Firefox), everything works every time.I don't know whether this issue has anything to do with routing, and I've no idea even what to look at next. The strange thing is, when I debug, the intro page doesn't fully load every time. Only about 1 in 3 times does it fully load even if I've just cleared browser cache. When I run it through fiddler, it fully loads and works fine every time.