Frequent Latency Variation

We got Xfinity almost a year ago & initially our connection was great. After the technicians installed the new Gateway things were great. About 5-6 months ago I noticed while online gaming that I started to experience latency variation more than I had in the past but it wasn’t bad enough to where I couldn’t brush it off. Then about 2 months ago my partner’s customer’s/co-workers started telling her she was cutting out frequently & haven’t stopped since. I’ve noticed this online gaming big time. I play a game where the games only last 5-6 minutes but I’ll experience latency variation 5-6 times in one game making it basically unplayable. We had poor internet at our last place so we purchased an upgraded speed plan that’s supposed to be super fast & allow up to 8 devices to connect to it. I usually game when my partner isn’t home, I’ll restart my router before playing every time, & pause all devices on my network & it still happens. Also most of the devices we’ve experienced latency variation on (Roku Smart TV, MacBook Pro, PC, etc) are connected via new Ethernet cables. When I try to use the Xfinity Assistant to fix the problem it detects that our internet is connected fine (which to a certain extent is true, the rate it cuts out is the problem) so it doesn’t even let me schedule an appointment with a technician. When I call customer service it just texts me a link back to the Xifinity assistant.  My partner’s job depends on our internet so we would really appreciate some help. Thank you!