Also limit your upload speed. Set it to about 80% of the total speed. That means you should set it around 20KB/s.
Finally, what trackers are you using? Public or private? If on a public tracker, then your download speeds may be low due to speed limits set by other seeders and the presence of partial seeders in the swarm.
I face the same problem with my MTNL connection. Ports are forwarded correctly.
I get good speeds with my net connection from the local cable guy.
The only thing I haven't tried is a different modem. May be the modem is choking due to the number of connections the Bittorrent client tries to open. I doubt that is the case though.