So I've got a desktop wired plus many wireless devices on my home network.

Wireless is fine.

My desktop wired PC has issue getting online. I've just done an install of win 7 for other reasons and I get the Internet but it is worse than dial up performance and I'm on fibre network.

I've restarted the router and changed all the lan cables with no luck.

It has to either be the LAN in my router OR my desktop motherboard.

How can I troubleshoot this?

my router stays on 'connecting' for IPv6 But IPv4 is connected, not idea if this is related?