You can see that one contact is not soldered properly, after i desolder this port, fix jammed pin and solder it back -> problem gone.
It also fix "fsc errors" on this port.

Tools needed: Soldering station

Made in Chine... oh no... Europe =)

Router 1:
Xeon 4 cores 2400Mhz
Intel Gigabit Desktop Adapter (82574L) PCI-E

<->

Router 2:
Core Q8400 2660Mhz
Intel Gigabit Desktop Adapter (82574L) PCI-E

Result:
rx/tx - 950000 pps