That's a common problem. Go into the settings of your bittorrent client and reduce your upload rate to about two thirds of your max capability. Usually fixes it.

The reason is that for every datapacket you receive, you send a small confirmation packet back to the sender. If you cannot send these back at a constant high rate, the sender will slow down your download to compensate. So you need to keep some of your upload capability free for these to flow freely. Most clients are set to use your full upload capability to share with however, which chokes your connection and ruins your download rate. So if you reduce the upload rate in your client you should get decent download speeds again.