Where do you measure those 50 Mb? On the ISP side, on the router, or on your PC?
If on the PC, you might have “acquired” some virus/trojan/etc., or if you have a resent version of some torrent clients, you might got “drowned” in their UDP (or even tunneled-ipv6) traffic.
BTW, if it’s torrent client problem (easy to diagnose – just kill clients in normal mode), using smartly the firewall on the router you can circumvent it: just block UTP (except port 53 for DNS) and ipv6 traffic. Torrents will fall-back to TCP connectivity, and TCP will not allow “drowning”, as I call it :o))
If you got a virus/trojan – scan and clean, but I’d better reinstall the OS…