Port forwarding issues. Ports forward briefly (10 -30 seconds) and then are closed

I have set up simple port forwards via the new Xfi app interface.  I am briefly able to connect and then the gateway closes the port and no future connections are possible. Rebooting has not changed this experience. I have seen other posts on the forums regarding this; often resulting in a direct consultation from comcast.  Please advise how I can proceed.