Hmm Cox are reknown for issues with p2p! I presume your incoming is ok, as in this test: Bug Report instructions & sample image Is the result True/False?

Are you using your D-Link in UPnP mode or manual port forwarding? I don't know how reliable D-Link's UPnP ability is. Since it wasn't available when I bought my D-Link 17 mths ago. (My model 504G doesn't support UPnP, in fact they stopped supporting my model altogether.)