Weird port forwarding problem

[cmichael]

hey guys, utorrent works fine at first. But after serveral hours, it seems that the port is not forwarded any longer.

Here is the thing. I'm using an ADSL connection and a router. I set my current computer as the DMZ server on the router, which means all the incoming connections should be forwarded to this computer. utorrent works fine at first. After several hours of continuous downloading, the green tick icon becomes a yellow exclamation point. I use the "speed guide" to test if the port is forwarded properly and it returns an error.

And the speed is reduced by 1/3 because of the blocking of incoming connections.

I have to restart utorrent (I choose to randomize the port when starting utorrent) to change a port which is fine again. I check the log of the router, it shows that the adsl has always been connected and I'm also prettey sure it's not a firewall issue because I have no firewalls enabled.

how to solve this problem?