@ Martin Blank,
OK, now that I have some personal time, lets see if we can work something out for you. I have to go to my real job tomorrow, so replies will be sporadic, but unless a war with Iran breaks out, I should have time daily to try and help you as Furry has asked me to. As I stated above, my specialty is RF with a minor in digital. I have used just about every major brand of networking gear out there. My kids have been pushing the limits of home networking for at least a decade. I have drawn the line though, if it requires SFP modules or even takes them, it is a little rich for personal gaming and media (and my wallet).
A few years back I purchased a new product that was not mature and it had some bugs that later got worked out through firmware updates, I have no doubt that your firmware still may have some bugs in it, it was just released 7-8 months ago. I have the Dir-825 which was purchased for it's IPV6 support, which is still stellar to this day, my router at the time was said to reset itself daily. My personal best on it was 6 months and it had to be reset because of internet provider issues. I do not use the RF portion of my Dir-825 any longer, it is not powerful enough to cover the area which I need to cover, but in the beginning I did use the RF section. I have to run 2 A/P's at home, we have too many things connected to WiFi and when the throughput of the WiFi gets saturated, games and media start lagging, so if you have 10 high bandwidth/packet rate clients connected to 1 A/P, that can be a problem and cause stuff to crash. Or gaming parties as my kids do. Want a nightmare? 5 Xbox's and a house full of teenagers on Saturday night, pray your provider does not go down!
I am major leaning towards firmware issues with your series of router just because I had similar problems that the owners of the Dir-657 are having, but I have no doubt that D-Link is working on them and they do read this forum, how often, I have no clue. My personal opinion is that RF should have never been added to these routers, if the internal shielding is not 110%, your going to have problems. My 3 kids have beat the heck out of my home network, and mine could be considered an "extreme" case for most home networks.
My understanding with your case, the WiFi works fine until you enable "N" mode? Sorry to be redundant, but reading through the 5 pages there was a lot of "me too's" from several people.
What WiFi clients are you running/what are you connecting to WiFi with on the other end, what mode of WiFi do they support (B/G/N)?
Lastly, are you interested in a workaround until D-Link fixes any problems we find? The workaround should NOT include turning "N" mode off, because of the Good neighbor policy required by the FCC, I would prefer that we try with 20Mhz channels only though(at first). This will usually get you 65-150Mbps depending on the WiFi client and the distance between the 2. My router will do every bit of 300Mbps with short GI turned on(270 without), but a lot of stuff chokes with short GI because of the quality of the clients radios can't handle a TX-RX transition that fast.
I am more interested in your clients then interference and router settings, though I may just need to verify some router settings initially. Simple stuff like, if you have no "B" clients, set the router to G/N only.