I used to have these issues at my old house with a different internet provider.
I did everything the net said. Including opening and forwarding ports. Static IP. Everything. Then I called my internet provider. Because I had a modem as well as a wireless router, I had to get the company to make my modem strictly a modem from their end. Then I had to set up my wireless router with all the port forwarding. Then it worked like a charm. Mostly. I still had to do the odd modem/router reset.
Since moving and changing my internet provider I have had 0 issues in the last 5 months. So. Chances are it's not your Xbox. There are a lot of holes in the entire network that could be the cause.
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