I am having similar problems. I used aMule 2.1.3 for months on Mac OS 10.4, no problems. I saw very healthy upload and download speeds.
Now that I've upgraded to 10.5.1, I get:
* fewer sources
* slower download speeds
* connection "arrows" will go from full green to green/yellow or yellow/yellow over the course of an hour
This happens with multiple servers, as I've tried to manually re/connect.
Overall, to put it unscientifically, files similar to the ones I'm downloading now have finished much quicker on 10.4. Mind you, I am getting SOME throughput, and two smaller archives have finished. But something is clearly up with Leopard.
w.