Is your device a Dell by any chance. If so, check to see if Smartbyte has been activated in a recent update. If it has - switch it off. If it isn't, is there any bandwith control software of any kind running unbeknown to you? Try a new Cat6a cable - they can "go off"! Do you have powerline adapters in the loop somewhere?
-------------------- Services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.
I've been unable to locate your account to run some checks so I’ve sent you a private message (purple envelope, top right corner) for some additional information to help me look into this further for you.