Intel pro 1000 gt issue

Hello,
Hey guys, I am new here.. desperately need a hand with my project I am undergoing at this moment. This is the scenario, have 4 machines set up. machine one is the traffic generator (D-ITG) and 4 is the traffic receiver. The 2 in the middle are connected merely to allow for some minor processing to occur to reduce traffic. The topology is bus to ensure no skipping occurs.. this is the toughie.. the two middle computers both have dual cards running pro 1000 gt adapters as well as the outer two.Speeds are meant to average around 768000 Kbps. However, the average throughput is about 198000Kbps after some more testing I noticed the flow between the dual cards on the middle two machines are restricting traffic flow. A network bridge is installed on both middle computer. pleasee HELPPPPP guys.. running out of time