Issue with DL speeds in large swarms only.

[theTREE]

I have done some measure of research on this forum but couldn't find an answer to my problem so I thought I'd ask. First off, I am not new to torrents, computers or networking so hopefully we can skip the obvious answers (port-forwarding, firewalls, uTorrent setup options, etc.).

The setup is like this... normal desktop computer running XP connected by router to cable modem. No issues with firewall or port-forwarding. Torrent downloads have been steady and consistent for years, easily able to achieve maximum speeds allowed by cable connection (obviously depending on torrent proliferation). uTorrent settings are set conservatively and the old rule of not allowing upload to exceed 80% of download capacity so as not to choke the connection is observed.

Now for the problem, lately, over about the last two months download speeds have been perfectly normal for torrents with a small number of peers... roughly say less than fifty. However, popular torrents with a very large amount of peers (often in the hundreds or thousands) absolutely never top 35 kB/s and usually stay solid at 30 kB/s. No matter how many active connections to Seeds and Leeches there are and without regard to the Availability of the files being downloaded. For instance, I currently am looking at a torrent that is just under 5 GB that shows peers as; Seeds - 27(498), Peers - 206(10874) and Availability - 65.998. This has been going steady for days without breaking 30 kB/s. Again, this problem ONLY affects torrent with large swarms, smaller torrent are not affected. Also, other methods of downloading files are not affected (i.e. http, ftp, newsgroups). At first I thought torrents may be throttled by the ISP, but I haven't found any other examples that are able to target specific popular torrents rather than just a sweeping range of torrents and similar connections.

Any help would be appreciated. TYIA