network port hangs

Network problem on Mac pros (2006 2.26 4core & 2008 2.8 8core)

Hi,

I have 2 older Mac Pros and I have been having identical problems on each machine...

I have been having a networking problem for some time now where my network connection seems to hang and switch to "self assigned ip". However it only happens in very specific circumstances...

Problem:

When large file transfers are happening over the network to a freenas network drive, the network connection will randomly hang until you reboot. This happens when transferring files from one network drive to another or from one disk image to another disk image on the same network drive. I have observed This problem on Mavericks, Yosemite and possibly mountain lion. There is no issue when running lion or earlier. There is also no issue on a 2010 MBP running Mavericks.

Things I have tried which have not worked:

1. Booting in safe mode
2. Copy files via drag & drop
3. Copy files via carbon copy cloner
4. Reset pram
5. Reset smc
6. Install a pcie network card which runs off of different .kext's
7. Repair permissions/disk
8. Try on Different user accounts
9. Create a new user account
10. Reinstall OS
11. Tried different network cables

Things I have tried which DO work:

1. Boot into snow leopard
2. Work from a 2010 MBP

Observations I have made:

1. It can hang after transferring anything from as little as 5gb or after as much as hundreds of gb's
2. It never hangs under any other circumstances, only file transfers between network drives or disk images on a network drive on Mac Pro running 10.8/9/10
3. I now have 3 network ports (2x on board, 1x pcie) when the connection hangs on, let's say port 1 the status changes to "self assigned ip". I can remove the cable and plug into port 2 or 3 and get a strong connection with DHCP assigned ip address. Put the cable back into port 1 and it gets stuck on "self assigned ip" again until I reboot. Put the cable back into ports 2 or 3, wait for the green light and then start a data transfer. Before long, that port will go 'dead' too.
4. As I can get it to work by booting into an older OS, I have ruled out hardware faults
5. As I can get it to work by using a MBP running mavericks, I have ruled out operating system incompatibility
6. I have also ruled out faulty cables/switch/router/network drives

Anyone have any ideas?

Many thanks