Why am I Paying For More and Getting Less

I am beyond infuriated. I switched from the 150mbps speed, which was very consistent, to the 500mbps speed. Oh I'm sorry, "UP TO 500mbps. Go to check the speed and I'm @ 70mbps??? WTF am I paying more for and getting less? I know they "say up" to but they aren't even meeting the speed at which I upgraded from which was far more stable and steady until I switched to this ridiculous "Panoramic" Modem. Well, COX on day 1 , you are in breach of contract....I will not pay you for an over 3/4 drop in speed. For 4 hours now the speed has remained @ 70mbps.... No Thanks. I need a guaranteed speed not *** guessing games!