Your requests might be queuing up on server before they are handled and then times out.
1) Try throttling in your WCF services.
2) Try using PerCall Use the following to remove the Session in your interface
[ServiceContract(Namespace="namespace", SessionMode=SessionMode.NotAllowed)]
Contract class implementing interface
ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
You should be able to enable tracing to see exactly what is occurring.