So, I'm going to suggest more pretty obvious and/or stupid things:
Have you installed any new soft (or changed some setups) just prior the problem ocurred?
Have you googled enough in order to find some people with similar problems/similar setup you have? Maybe adding the name of your ISP should focus better results.
Can you try torrenting at a friend's house and see what happens?
Did you try uTorrent? (Small memory footprint and demands few CPU). Although I don't know whether is there a version for Mac.
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