Very weird. Can you try Acquisition (find it on macupdate.com) it uses the LW code with another gui. Maybe some settings are broken.
What is your ISP? Maybe they use sandvine or something to throttle P2P speeds. Call them about it. Also change the default port for LW from 6346 to anything you want, this may help. I really think this is an ISP issue.
Désolé
