Ok the reason is that I currently have two broadband lines, both with BT. One is unlimited and one is 10 GB capped. I use 2nd line to play Sensible Soccer online occasionally, taking part with online league etc but it always throws up strange things at online matches.
I believe that high error rate only started on both lines after the installation of 2nd line/broadband last December. When it was being installed, BT engineer said something about a fault on the line down the lane needs fixing and that broadband can not be activated until a few days later. A few months later, BT engineer came and checked the line over and said it was all fine. Secondly, both sockets are masters and are next to each other and both have ADSL faceplate and both are connected to routers directly. Only one of them is connected to a telephone, thats it.
I guess I should switch to cheaper ISP, seeing that it is not going to improve anyway.