if theres a sudden problem after months of 5meg connection why is the answer to cap the speed and not sort the problem
i'm not sure what you expected sky to do, i would assume when you spoke to them they asked you to plug into the test socket ? if so and you still had enough line errors the only thing they can to stabilise your line is to cap you

the sudden connection change could be down to lots of things, if you had been using an extention lead this could have packed up, weather plays it's part in a lot of problems also