I have been experiencing similar speed issues with Comcast in the US (after a large snow storm but persisting for a week). The speed measured on the same site and using the same browser changes over time. I have called the vendor and we have had technical service visit. They verified this was not due to my equipment and finally confirmed they have some equipment problem upstream of my house. It took many calls, and calls of other subscribers of Comcast in my neighborhood (we all share the same node and should all be having the same problem). My problem was about every 30 minutes the browser speed would drop to nothing. Sometimes I could not even run the speed testing because I could not browse to the web sites. After 10 to 15 minutes speed would resume, at least I could test the speed, and it often was low.
In addition I also tried other sites for speed tests. I tested here at testmy.net as well as using the other site you showed in your note. Now that software allows me to pick my server and I chose one close to my home. With testmy.net the server is further away so I get lower numbers than with this other service. (I am new to this board but I understand we are not suppose to mention the actual name of other services for testing speed). So there should be some differences based upon the distance to the server.
But your table of data from some testing showing large speed difference should be enough to convince your vendor to look into their equipment. Comcast has the ability to "look at" my node from their main office. They can tell if my cable modem is behaving correctly. I am currently under "constant monitoring" - in theory they should also see these service drop outs. Also I got the name of a senior technical person I can call for updates and to leave messages when I experience poor connection speed (rather than waiting in the standard queue).
While I will admit it is possible that my equipment could be an issue with speed, I also think they like to blame the user and unwilling to accept their equipment is bad or failing. But after rebooting my computer, my router and my modem - the problem persisted. At that point it is most likely their problem. I became very firm with them. It also helped that a neighbor was having the same issue. (They think a single complaint is probably due to that one person's equipment or computer, whereas two households might be an actual problem).
