When downloading demos and occasionally when playing multiplayer, my Xbox 360 is causing my Internet to disconnect.

It'll be downloading away, then stop and after a few seconds disconnect from Xbox Live, a check of the event log in the FritzBox shows the Internet was disconnected and then reconnected at the time the download stopped, but the Xbox doesn't reconnect automatically and I have to reconnect to Xbox Live manually. This only ever happens when I use the Xbox. I've been using it most of the afternoon and have had about a dozen disconnections so far, pretty much makes it unusable unless I'm playing single player. I'd say about every 7-8 minutes this happens when the Xbox is actively accessing the network.

It's normally connected by Ethernet, but whether it's wired or wireless it makes no difference.

I have tried putting the Xbox in the DMZ, I've tried disabling IPv6, but nothing makes a difference.

I've used the same Xbox with Orcon, Telecom, and Vodafone with the same network setup in the past and never had this issue, so I have to assume it's something related to the FritzBox.

I'm on Snap's 200/50 fibre plan.

Any suggestions?!