NIC Teaming Limits

We have 5 servers and each server has 4 NICs teamed together. All four ports for each server are configured to use LACP and port channels and LACP has been configured on the Cisco 3750. When using iperf, from 4 different servers and point them all at the 5th server, we cannot get 4 Gbps to pass. We end up with 3 Gbps. Based on our monitoring a single NIC on the "receiving" server gets no traffic sent to it. The other 3 NICs on the "receiving" server are running at 1 Gbps. We have tried lots configurations the Team side and we have even modified the load-balancing algorithm on the cisco side, but it doesn't seem to help. We can manipulate the symptoms by changing the load-balancing algorithm, but the underlying 3 Gbps limit still remains.

Any help would be greatly appreciated.