Burning Typhoon on 21 January 2015
It's not drastically changing from match to match, it's drastically changing from moment to moment. Call your cable company, perhaps your wiring is going bad. I had dips yesterday down to 1.5MBPs, and they came right out today and fixed it in about an hour.
It could be something else too, though. Just... run some tests, call your cable company, and talk to them, and see how it goes.
