I have the same DMZ, port forwarding, and lock ups that others have documented. I have made several valuable and helpful posts about this - including that disabling DNS Relay resolved the lock up specific issues, but still makes the router useless to me if I cannot use any of the advanced networking like DMZ or port forwarding. To date, D-Link CHOSE to kill that thread completely.
I do believe we need to take further action. Negative word of moth does wonders and seems unfortunately necessary at this point. Regardless of price, we all spent money for a product that simple does not do what is expected. It used to, until D-Link provided additional features (that it advertised) and simply do not work. I upgraded my firmware because I had a need for removable media. I was FORCED to upgrade further because no-one would addmit or address the lock ups many of use were experiencing. It was that second firmware upgrade that killed my port forwarding and that should have immediately been adressed by D-Link. This was their flagship router for this segment of the market and they seem to have abandonded it and their customers who bought it. Looks like it is back to Cisco/Linksys for me - AND, though this router was for my personl/home use, I do use routers and switches in my everyday life in the IT business. Guess you can bet where my future purchasing dollars and recommendations to other customers is going!