X550-T2 reverting to 100Mbps on 1Gbps link.

[SpuddyUK]

Hi,

Recently upgraded my opnsense hardware to 10Gb (X550-T2) so that my inter-vlan traffic doesn't impact my 1Gbps WAN speed. However, I'm finding that periodically my 1Gbps connection to my fibre ONT (opnsense WAN PPPoE interface) is reverting to 100Mbps. I have tried "default", "autoselect" and "1000baseT". All seem to connect fine at 1Gbps but demote to 100Mbps after varying periods of time, sometimes days. A reboot of opnsense usually solves it until the next time.

How best can I troubleshoot this?

Thanks
S