Internet speed or connection dropping

I have an XB7 CGM4331COM modem with a plan of up to 1200 mbps. Service was fine for about the first six month.  In the past few weeks, my download speeds at night drop from 900-800 and fluctuate wildly from normal to 100-300 or even zero. This happens on an almost nightly basis.  I have tried restarting the modem during these episodes and it does not resolve the issue. 

A technician came out and said that he could not identify the problem without seeing it happen, and the issue doesn't result in a complete outage that registers in the service logs.   

The technician suggested that the problem might be with the connections and looked at the connections from the modem to the devices inside the house.  He suggested we plug our ethernet connected devices directly into the modem bypassing switches, which we did. He did not look at or test the coaxial cables either inside or outside the house.  He noted that further service calls that do not identify a problem could result in $100 charges so I do not want another on site technician without some assurance they will do more than check their tablet for errors.

Last night the problem lasted about an hour and at times resulted in speeds so low the xfinity speed test page could not load and at times returned upload speed and latency errors.  

I have been happy with Xfinity up to this point but am very unhappy with the response to this issue (which others have experienced based on the posts here and elsewhere). How do I escalate Xfinity response?