Inconsistent Download Speeds Which are Much Lower Than Plan Calls For

I own my cable modem and WiFi router. Both are rated for speeds up to 1000 Mbps. My Xfinity plan calls for a download speed of 1000 Mbps and an upload speed of 20 Mbps. Up until about a month ago, I generally had download speeds of between 300 and 450 Mbps even during peak usage hours. At that time, Xfinity spent two days doing something to the system on our court. After that, our download speeds abruptly dropped to between 40-60 Mbps on average, sometimes going as low as 6-8 Mbps or cutting out completely. On rare occasions, we will get download speeds of nearly 800 Mbps lasting an hour or more. At first I thought and hoped that it was just a problem with the repair work or whatever they did and that it would soon sort itself out but it hasn't.

I have checked and rechecked all connections. My WiFi router is a Netgear Nighthawk Model RAX54 and is about a year old. Its firmware is up to date. My cable modem is a Linksys Model CM3024 and is a couple of years old. I have rebooted both several times but no real changes to the low and inconsistent.

In the decades which we have been Comcast/Xfinity customers, we have never experienced any significant problems until now. To say it's gotten more than a wee bit frustrating would be an understatement especially when trying to watch streaming fare.