Constant upload packet loss past XB8 gateway, signal levels normal - causing servere jitter in game

My wired PC sees about 1.5–2.5% packet loss past my XB8 gateway, which causes jitter in online games.

• Ping to the gateway: 0% loss. Ping to 1.1.1.1 and 8.8.8.8: both drop packets at the same moment, about 3 times a minute, with no increase in ping.
• Already tried: restarting the gateway, reseating the coax, and moving the gateway somewhere cooler. Loss improved briefly, then came back within about 20 minutes.
• Signal: downstream power 1–4 dBmV, SNR 39–44 dB; upstream power 43–46 dBmV. Uncorrectable codewords are low.

The signal levels look fine, so I suspect noise on the upstream side. Could an Xfinity employee check upstream SNR and codewords for my modem and node?