X6 R8000 keeps dropping to 100Mbps wired

It’s a virgin media Hub 4 it’s connected to. Changing cables to Cat6 isn’t an option (Cat5e perfectly capable of carrying a full duplex gigabit signal over 100m my run is only around 20m with shallow radius corners) it would mean having to rip plasterboard off of most of the bottom floor of my home to get from where the AP is to the cable modem. But the cable is known good with continuity between ends. When the problem occurs the link between modem and AP (WAN port) is the only connection to remain at 1000mbps, all the LAN ports drop to 100. Tested all cables with Ethernet tester and multimeter all good.