I had a sittuation like yours, and how I fixed it was upgrading my internet connection. Because dsl does not have static IP usually drops down a lot, when its bandwith is being use at its full potential. In other words, the modem+router start updating its IP address, to gain more bandwith. Bitorrent stops the download, until you refresh the connection again. Having a metal roof top, also means that a lot of the upbeams are metallic, and that is also a problem. You may need to use a lot of AP, so the signal would be stronger.