Gigablast Speeds

We have had Gigablast for I believe 2 years now. Yet not once over these 2 years have I seen the speeds go over 500-600 Mbps. For the past couple months or so I haven't seen it over 300. Yet were paying a ridiculous $182/mo for the plan quoted at $120. 
I am using a high end computer with a cat 6 ethernet connection from the Cox Panorama modem.
Yes I have tried restarting the modem and all the other cookie cutter things tech support tells you to do.