It's difficult to follow what's going on here. Can you provide more detail, or confirm the following?
>You are trying to authenticate Windows clients with the windows supplicant. (You are not testing iNode)
>Authentication passes and Dynamic VLAN assignment via UAM access profile works... initially anyway
>However during that time... the interface in windows (ncpa.cpl) goes from an enabled state back to attemting to authenticate, then subsequently fails.
I would check a couple of things.
1. From the network connections windown (run menu 'ncpa.cpl') right click the interface --> Authentication tab ... I'm not in front of a client machine, but I believe it's the advanced button where you can specify to use user authentication (vs computer or user authentication). The default is both, it may be that the system passes one and fails the other. Changing this setting will narrow the troubleshooting.
2. Run Wireshark on the IMC server or UAM sub-server, (whatever server UAM is running on) setup a filter to display radius. Look at the radius requests coming in when you are testing the clients, and the radius accept, radius deny messages to see what kind of auth is attemted, what the error messages returned are, etc.
Good Luck
PL