Someone suggested that I use a CAT7 cable but I seem to recall the Fritzbox Flat Cables are CAT7, (I have never had any issues with them), however, I did try a three meter CAT7 Cable from Amazon Basics that I had in my garage and I tried a new CAT6A cable, (that I made up myself for the test using a length of CAT6A cable and a couple of CAT7 plugs that were left over when we had the house wired for ethernet throughout), but that made no difference. - Once raised as an option; it did need to be considered because when I plugged the PC directly into the ONT, (prior to my making my first post on this thread), I had simply moved the ethernet cable from the router WAN Port to the PC RJ45 Port).
Someone asked if my equipment was capable of the speeds but I did point out in the earlier posts that it does sometimes run at over 900 Mbps , also my my PC and its ethernet card are rated to suit the speeds, (prior to my making my first post), so that is not an issue.- I use Windows 10, I keep it updated and I do not tweak it; I use Kaspersky Total with its tools to remove any rubbish. - I have tested the line on other equipment, including an almost new Xbox One X and the reported speeds on the Xbox One X vary in time with those on the PC.
Soon my circuit is to be given Elevated Traffic Status as that is what I pressed for as a first step because the general consensus seems to be that it is all down to contention and I am optimistic that it will cure the issues.
We do need it cured since, as can be seen from the above speed tests, today it is just more of the same.