Consistent low upload speed and Ping Jitter

I've been dealing with consistent low upload speed and ping jitter issues for over 3 months now, with upload speeds dropping to 0-10mbps during day time hours and ping spiking well into the hundreds. I have already isolated my modem itself as not the issue by connecting directly to it using my computer, bypassing all routers and switches, and it still retains the same speed and latency issue. I have inspected coax lines, and ran coax directly into my modem, with no splitters, and it still retained the same issue. I have also been in contact previously with Xfinity in regards to this issue, and have had service agents sent out to my location. 3 separate agents have come to my house, and I had been informed that a ticket had been placed, and the underground cable was to be replaced. I received an email regarding this 3 months ago, but since then have had no followup from anyone in regards to this issue. The equipment I use is an Arris SB8200, and I do not get 35mbps or even close to it during the day.