Poor internet service

We pay for a ten meg adsl line it can hardly run one tv without buffing when u call custermer care they say u get from 5 meg to ten meg but paying for a ten meg lineRover R800 a mnth if i do speed tests its usually just above the 4 meg speed this is corupt many calls to fix this same story every time its not garanteed 10meg.. why charge for a ten meg line then charge according to what u delivering very anoyed