Slow WAN Speed over DS3

I have a WAN connection setup from NYC to LA with a shared IP address for load balancing. This line has a dedicated 12 MB line but we are having troubles in trying to get it to connect and transmit at any speeds over 3 MB. I have tried passing actual files over this line and received a top speed of about 3.15 MB per second and upon doing an iPerf test which passed only TCP packets, I still only see an actual bandwidth of 3.0 MBS. Any ideas of what might be causing this kind of bottleneck?

I have been in contact with the owner of the DS3 as well as Cisco about the routers in between, and neither can see an issue with this speed problem. They say everything should be at speed…

Any help out there would be great.