Tech came out service was worse with new modem only seeing about 68mbps speeds

Recently a representative call and said we could get a "free" upgrade to the xfi gateway from my old Arris modem, because it was not able to handle the new speed..

We were upgraded from 800mbps- to 2000mbps and were sent a "new" xfi modem to handle the speed

 Unfortunately it was much slower and less reliable than my old modem.  I called tech support and after a couple of hours trying to fix things remotely, the representative said they would have to send out a technician to fix the problem and we would not be charged.  This did not happen, the tech replaced the modem with a new one, speed did not improve, in fact was worse and we were charged $100 for "professional installation"   I wish there was a competing cable company out here, even though I used to swear by Comcast, now the service is terrible.... What can I do to fix my speed and being charged for a service that did no better than the one I installed myself that I purchased?