200Mbps to >5Mbps 30 Minutes After Modem Reboot

My Xfinity 'Blast' tier connection consistently slows from 200/10 to 5/1 within 30 minutes of rebooting modem. Rebooting Netgear CM400 modem resolves, but issue returns within 30 minutes. Modem is directly connected to coaxial cable from the Comcast box outside.

 

Any ideas? I'm getting the impression this is a Comcast/XFinity issue outside of my house OR the CM400 modem.

 

• Everything seems within spec: https://forums.xfinity.com/t5/Internet/ANSWERED-Tips-for-troubleshooting-your-Xfinity-Internet/ta-p/2941127#
• Frequent Error in Modem Log: No Ranging Response received - T3 time-out
• Modem connection stats: https://imgur.com/a/7BfbIdm
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