Troubleshooting 2gb speeds

Upgraded to 2GB internet service but speed no better than 900mbps and now very erratic.  Speed ranges from 120 mbps to 976 mbps, nowhere near 2GB.  Using wired ethernet connection direct from new Xfinity gateway to computer with CAT6A cable.  Tried WiFi but experienced the same results.  On an amusing note, my computer keeps telling me its capable of much faster speeds that my internet service is providing.  I have tried swapping cables, restarting gateway etc.  Service is much worse since upgrade.  Any suggestions?