Does anyone know anything about how to get the AD Schema to refresh under IIS6 on Windows Server 2003 using Windows Authentication?

Specifically, we get the error when using the following code to get the properties of an AD User:

We've been struggling for days on this, and the problem seems to be somehow related to the way IIS connects to AD using Windows Authentication. Can't figure out if it is a permissions problem or what.  I've checked the impersonation and even if we impersonate a local admin account for w3wp we still get the error.  If we logon to app using Forms Authentication, everything works for a while under Windows Authentication until  1) the AD Schema Cache refreshes or 2) we recycle the IISAdmin Service.