First off, you came to the right place. Welcome.
What is your network layout as far as routers and or switches.
I would as you say " light a fire " , chances are, there is a line issue. Your DSL speeds should not be fluctuating that much unless there is other internal network activity " slurping " up the bandwidth. You also have to keep in mind as to the distance from there router that you are. They cannot guarantee speeds when you are over a certain footage.
If the lines inside the house are properly installed, and clean, and you know this as a fact, as the ISP is not liable for this, your phone company would be if you have this insurance.
Just keep on them about this, persistence pays off.