Slow ethernet and killing lan

Im having some trouble with internet connection, normal surfing the web works fine. when downloading torrent the hole home net almost dies, terrible slow speed and the torrent are not using my hole 100Mbps, the speed jumps up and down, same jumping when file transferring to local nas. the ethernet card is NetXtreme BCM5762 Gigabit Ethernet PCIe, have the right driver installed, the system was running as ubuntu server resently and working fin.. Im stuck, anybody have a tip?