Doing the exact same with me mate, It works totally fine but when I connect it to a Ethernet it connects then disconnects again. Pethetic now!
No I didn't have to buy a new router in the end.
I replaced the ethernet cable and went through a couple of power reset cycles of the router.
One cycle, I unplugged the ethernet cable, turned the power off, left it for 5 minutes, turned it back on...then when it had a stable internet connection, plugged the ethernet cable back in.
This gave more stability however sometimes still ended up with the issue described in this post.
I then performed another power reset cycle as above and then this seemed to alleviate the problem.
Although I can't confirm this, from my experience I've had, I believe as the router dynamically assigns IP addresses, the router remembers devices footprints on IP addresses...so when a new device is plugged in and dynamically assined an IP address...the router has trouble identifying that it is a new device and therefore has this problem. That's the only remotely logical thing I can come up with from what I experienced.
Also, when you plug in your Xbox, leave it, don't do any tests or try and play a game, I left it plugged in for some time after doing the above and this helped it gain a stable connection with the router I believe. I've noticed with this router than when you plug in a new ethernet connection and try and perform tests, it leads to more instability and dropped connections...I have no idea why.
Hopefully my reply below will help but no guarentee unfortunately. Good luck though.