I have a 7480 [non-TB] connected to a WD15, that up and loses its Ethernet connection. The BIOS, associated drivers and FW, are up-to-date, to the best of my knowledge. Moving the Ethernet cable to the laptop, itself, serves as a workaround.
There's a firmware update specifically for the Ethernet controller in the WD15. For some reason it's no longer listed on the WD15's Downloads page on support.dell.com, but the direct link is: www.dell.com/.../driversdetails