WCF Intermittent 403 client authentication...scheme 'Anonymous'

Question

Hello.

I am not much of a developer anymore.  But am in a client support role.   We have a WCF web service that runs on .Net 3.5 and Windows 2003 server.  Client machine runs on the same. Client machine is a web app. ASP.Net.  We use transport security with Certificate authentication.

The Web Service call works for a while and then gets hung up with ...

The HTTP request was forbidden with client authentication
 scheme 'Anonymous'
 

Now . although it seems like a server problem, the issue is resolved (short term) by an application pool restart on the client side.

If it works sometimes, it seems all is set up correctly , permissions, cert install etc.

But I can't get anywhere with my support ticket. 

Any guidenace on where ot direct the problem would be much welcomed.....thanks