No LAN link with E6530 on E-Port Plus II when docked

Hi there, i have a strange returning problem. I have been using my brand new Latitude on my dockink station for 6 months without problem. But one day somehow the LAN link was dissapeared, the LED was blinking on the switch. Once i undocked the laptop then the internal ethernet port worked fine. First i changed the cable, the switch,  then tried to boot with a Linux live cd up without luck. Finally i tried my docking station with another laptop (6520) and it was working fine. I called the service who made a mainboard replacement and the problem was fixed. It was working for less a week and started again from the beginning. The problem is the same, No LAN link, mainboard diagnostic did notice nothing. Internal LAN connector works fine with the same cable. Docking station has link with an other laptop.  Somewhere i read that these LAN devices are able to going down to deep sleep mode and somehow never coming back. However it is  contracticting  if even that doesn't work after reboot. Any idea? Shall i call the support again? Thanks