first off, you said you are using another computer as a router? i guess your using internet connection sharing in windows? if so you might wanna buy a hardware router, around $45 for a good one, sometimes $25 on clearence.

try the following if you can.
1. find out your fastest upload speed, and set bit torrent to limit upload to around %80-%90 of max.
if that doesn't work
2. install a bt client on the main computer and see if the problem exists there, again limit upload on this computer.

most likely, if your using internet connection sharing, that's the problem.