WMS & Authentication

I published a WMS out of AutoDesk Infrastructure Map Server (AIMS). I can connect and display it's layers and data using Openlayers, Gaia, Qgis and Mapguide Studio with no issues and no forced credentials. As soon as I preview it in ArcMap it requests authentication. I provide admin credentials (as I am the WMS publisher) and the authentication box pops up over and over again. It reads the capabilities and provides a list of layers but won't accept my admin credentials.
Any thoughts?

Thanks in advance!