Xf1

A few months ago, I swtiched from full comcast cable and internet to internet only. I chose the Extreme Plan-300 Mbps and added Fx1. My speed to stream is averaging 180 Mbps and we are getting constant interruptions to the streaming. We are still using the modem we had with full cable and internet. Were we suppose to get a new modem for XF1?  I feel like I am paying for something extra and getting nothing..Thanks