Slow internet speeds around 8PM every single night

I pay for 400 Mbps download speeds. I have been on the phone with Comcast support every night for the past week going through the same signal resets. I have escalated the cases several times and had a tech come on-site. The tech fixed some issues he found with the coaxial cable in the house and we also installed a brand new Arris Docsis 3.1 modem that I purchased since support said it was my modems fault. We thought it was fixed but like clockwork, in between 7-8PM. My speeds drop to about 4 Mbps down and .5 Mbps up. I understand these are peak hours and network congestion may be a factor here. I have gotten texts and calls in the middle of the night that says the escalation team found the source of the issue and corrected it and closed the case. I am trying to get someone to please take a look at this as I am at my wit’s end.