I am also having a similar issue, except my main OS is FreeBSD 7.3. I have tried using Ubuntu 11.04, which did not rectify my situation. I was wondering if anything has changed, be it replacing your cards, upgrading your OS, or if your problem has just gone away. I also feel that it's a hardware problem and after weeks of troubleshooting, to no avail, I'm tempted to RMA it. The following is my post, located at http://communities.intel.com/message/134451#134451
I purchased a PWLA8391GT NIC recently to use in a NAS to improve my performance.
My set-up is as follows: ASUS M4N68T-M VS Motherboard, AMD Athlon II X2 215 Processor and the OS is FreeNAS 0.7.2 Sabanda (revision 6567) running on FreeBSD 7.3-RELEASE-p6 (revision 199506). I installed the card and used iperf to test raw TCP speed between my clients on the network.
When using iperf, with my client computer sending data to the server, I am getting an average speed of over 850 Mbps with a total transfer of 1.02GB in 10 seconds. However when sending data from my server to my computer, it is apparently limited to only 319 Mbps transfering 382 MBps. I downloaded the kernel off the support site, removed the old one and compiled the new one with no difference in speed or performance. If I use two client computers with iperf, not going through the server, I can get full speed (meaning it's the NIC causing fault, not a switch or computer on the network). It appears to be the card's upload speed, limited by hardware, not software/drivers.
If anyone can help, it would be greatly appreciated! Thank you!
UPDATE: I comiplied the driver and found no difference in performance or speed, most likely it's a hardware issue, either caused by the card or motherboard.