Connection to 2 servers -> router stalled!

Hi,
Could someone explain to me how can this happen and where i could find the solution to the following problem?

When i try to connect to 2 different FTP servers to download from one and upload to the other, my router which is a 3com 3CRADSL72 (Bios: SMC) and to which i am connected through cable get stalled and has to be cold booted!

Neither Emule nor Bittorrent give any problems (if related anyway). No, they are not running at the same time

Any hints or how to put the right parameters in the router's bios?