you may have port forwarding problems. turning off your firewall works but you can just forward the ports. search for it, there should be 10000000000 threads for it. or you may be setting your upload rate too high and it might be working your dl speed. there could be other factors too obviously.

anyway, this is the one thing i don't like about bt. the speed ALWAYS varies for me. it's usually in the 200kb/sec to 250kb/sec for me (for most anime downloads) but for some reason when i dl certain files (especially big files like movies), i can't get anything above 20kb/sec. and yes i have my ports forwarded and all that. maybe someone with more knowledge can answer this.