Very Low Download Speeds 1 GB Internet Connection with XB8 Modem - No Human at Xfinity has been able to fix this.

I am an EE with a 1Gb Internet Connection XB8 modem.  Currently Max download speeds vary between 150Mb to 250Mb.  I had his service for 17 months with no issues.  Download speeds were 850Mb and upload speeds were 20Mb using multiple devices on the network.  About 2 months ago, Xfinity had a major equipment problem in my area, and we lost service for a day or two, followed by two weeks of intermittent / random downtime interruptions.  Finally after about 2-3 weeks the connection was up and connection was reliable.  HOWEVER, ever since then my download speeds have dropped to 250Mb and will not improve.  Used Xfinity app to troubleshoot, chatted multiple times with customer support, finally Xfinity sent a tech after several weeks.  Tech came and replaced the XB8 modem, checked all connections.  Speeds immediately jumped up to 850Mb again.  The Tech Left. THREE HOURS LATER SAME PROBLEM RETURNED.  Xfinity is at a loss to fix this issue which coincided with a Xfinity equipment failure in the area.  In engineering school we are taught "what changed?"as the FIRST step to seek a root cause.  I know what the change was, however no human at Xfinity can solve this problem after 2 months of trying.  Again, I am an EE and very familiar with the technology here. It is an Xfinity issue at their switch (probably involves their equipment outage a couple months ago).  I am about ready to chuck it all and go elsewhere!! I am patient but this is ridiculous....  It must be a provisioning problem, or the connection is being throttled, or still equipment switch issues at Xfinity.