Extreamly Slow TCP connection with IP of named instance on SQL Cluster

Hi,

We experience some strange problem with our SQL Cluster. First I can't connect to the SQL Instance using SSMS.

Using wireshark showing a lot of TCP Retransmission. I using iperf to test the speed, and notice that with the ip address of the Cluster host, the connection is good. But using the Ip address of the Named Instance of SQL, the connection speed is very bad.

Hope somebody can help me figure this out.