It does sound like an ISP issue, often its possible to UL or DL from other users on the same ISP this would explain why the odd file gets through.
First thing to try is to change your Winmx ports, perhaps add a 2 to the front of both the TCP and UDP, dont forget to make the relevant changes in your FW/Router.
You may have to spend sometime experimenting with ports, you may be wasting your time, you may not, it might also we worth trying to identify some other users with the same ISP and ask then if they have found a way round it.