Not getting gigabit speeds with CM1000

Had Comcast's gigabit internet service installed a couple months ago. Service is fast, but nowere near gigabit speeds (not even when connected directly to the CM1000 with a gigabit-capable device.  I've seen occasional speeds as high as 700Mbps, but average has been about 400Mbps.  Comcast just wants me to get their gateway to "fix" the speeds, even though the CM1000 is Xfinity-certified to work with their gigabit service.  Help!