Readynas102 strange network fail

Suddenly, without any changes to the config. of the nas, the network, or the pc, i can no longer move files around in Win7 explorer.

When i move a 4.2 Gb moviefile from pc to nas, it starts transferring at 30-40Mbs, i can see the new map being created in the nas/videomap, then
the transfer slows down to abt. 10 Mbs, the progress-bar turns red and transfer stops, reporting provided networkname no longer available, or cannot find network path.
At that moment abt. 2 Gb of the file is already transferred to the nas, so it cannot be a matter not having permits/access at all?

If i try same from other pc (xp) same thing happens.

After the transfer freezes, the 102 cannot be reached in any way, i have to pull powerplug and reboot to get access again.

No reports in NAS log, no errors on hd's, plenty of space on them left. In order to exclude the network with gs108t-switch, i connected nas direct to the second ethernetcard in pc;
nas showing up as usual with all shares, but same problem transferring files.

Even after extensive reading this forum, cannot find solution, and however unit is only a week old, get the feeling there is something wrong with it's networkcard??

Hope someone has suggestions?

Edit:

Networkcard of nas seems fine, pulling files to dropzone work (form win-explorerscreen to nas screen "drag and drop files to upload") 1 Tb in 3 mins

App. win7 is fooling around?

Edit: problem re-occured with file-transfer as above mentioned: tryed to transfer 8 Gb of files, after transferring 3.27 Gb (13 minutes) "connection lost" ?????? resetting with powerplug again....