Which clients are they from. If they are RAZA (Shareaza) then don't be surprised about 0 to 1 KB/s downld speed. Raza users use much or most of their upld bandwidth for a different network where they can only downld as fast as they upld. Some people have refered to raza as leechers, but it does depend. You can get generous raza users.

Check the connection type of the slow downlds. Is it modem for any LW users you're sourcing from?

I can only suggest you keep attacking the issue with trying to find more sources for those slow downlds.