-------------------- 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.
So PC has been connected to the router for 4 months directly. Very rarely have any issues except for occasional disconnects every now and then (not sure if that means anything?). Speed tests have been excellent when tested.
I agree with @Roger_Gooner: try a different cable but if buying a new CAT6 patch cable, especially one as long as this one, be sure to buy one that's certified as CAT6 e.g. from a network cabling specialist such as cablemonkey. (NB I have no connection/affiliation with them whatsoever).
I read an article recently in which an American cable vendor tested and compared a variety of CAT6 and CAT6A cables bought from a variety of sources to see how they performed against spec. Only 10% passed unreservedly. Of the 10 longer cables they tested (50 foot) only one passed marginally at CAT6, another tested OK at CAT5E and all of the others failed to reach even that standard.
Also, when running CAT6 cable make sure that it isn't kinked or bent sharply anywhere along its length. This can degrade its performance or stop it from working entirely. I know this to my cost, having had to re-route a CAT6 cable under my livingroom floor which stopped working entirely because it had become caught around a central heating pipe: freeing the cable reinstated the link.