I have tried to change the port to 5000 and I have the same problem. If an other process is running, xine for example, then the download rate is going down completely (while upload works well !). I have got the same problem with Mandrake 10 beta1 and therefore I switched back to 9.2 waiting for the release version. I presume a scheduling issue with the new 2.6 kernel.
Now I have tried MLdonkey and it works pretty well, even on mdk 10, with much higher download rates at about 50 Kb/s sustained and what is more interesting I have NO MORE an HIGH CPU ISSUE ( about 90% of total CPU time after few hours with aMule and xMule). I've got the high CPU issue over month and I've tried all tweaks without success.
The MlDonkey background process (without the GUI) needs less than 1% of CPU time (AMD Athlon +2200) at full download speed (50KB/s) .
If you meet the same high CPU issue like me and can't deal with it successfully, I highly recommend you to switch to Mldonkey!