After a while downloading torrents my network card stops working properly
Solution 1:
Sounds to me like your connection is being saturated and you need to adjust your uTorrent settings accordingly to allow some 'breathing room' for your connection whilst downloading.
This guide is written for Vuze but with a little bit of research you can adjust the settings so that they apply to uTorrent:
http://wiki.vuze.com/w/Good_settings