Why do my LDAP calls occasionally fail with these messages?

I am developing an intranet ASP.NET web application that's configured to use Integrated Windows Authentication. My web app needs to check user entitlements using LDAP to get user memberships, etc.

The issue I noticed today is that, when I run my we开发者_JAVA技巧b app using IIS, sometimes the LDAP calls fail with strange errors, such as "Unable to contact the server..." ... or "...that domain doesn't exist..." (something along those lines, anyway). However, if I run my web app using Cassini, I don't get those errors.

If IIS is the issue, why would it fail some times and other times not? What does Cassini do different that IIS is not doing?

I'm using IIS 5.1 / Windows XP / Visual Studio 2008

Why is this happening, and how can I fix it?

Probably something to do with the fact that when you're running Cassini, it's your domain account that's querying the LDAP. It is strange that it would work in IIS sometimes though, and not others. Is the server registered on the network? What system account is IIS running under?

@James Johnson: I figured it out thanks to your comment, which got me thinking about my domain account... Finally, I found this post which points out that in order to connect to LDAP one needs to set Impersonation=True in web.config. It was working intermittently this morning, I think, because I was using Firefox instead of IE, which required me to enter my domain username and password.

The other strange thing was that my app worked fine in our development and stage environments even though my web.config is the same for all of them. I suppose the guys in the hosting group have this set at the machine.config level.