Speed issues only at certain times

Hi, I am posting this here in the hope someone from Comcast will address this. I have Comcast 2100/300 service. In short, mostly at night (but sometimes in late afternoon) download speed from the router or a direct connected/ethernet MacBook Pro starts at 2300-2400, then slowly drifts down to 1700-1900. In the morning especially, it is rock solid at 2280-2350 Mbps. To me, this is indicative of congestion or shaping upstream of my house (node/service-group/CMTS side) than to anything on my network.

I am not sure if Comcast employees frequent here or not; I had a long conversation with Comcast, on reddit, and they refused to reply anything other than "we'll send a tech to your house; access will be needed to the cable outlets and the technician does not move furniture. Your services may be interrupted while the technician repairs and verifies the signal before leaving…" They gave no indication they even read or understand what I posted (which I added here below, so you can see for specifics). This is NOT an inside-the-house issue.

Now I have no connectors other than at the demarcation box that they installed outside, and the one into the modem - straight shot, no splitters in the house, cable inspected for nicks/tears/extreme bends, connections tight. As you can probably see, my line is near perfect as relates to power levels, SNRs, lack of errors, and any concerning log entries. I have been tracking detailed modem information for about 2 weeks, but I have 'eyeballed' the power/SNR/log entries daily for a long time.

Anyway - here is my data and my request for some upstream inspection, in the hopes there is support to be had here. Thanks!

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I’m on the 2100/300 plan with a Netgear CM3000 (DOCSIS 3.1 mid-split, 2.5G port) and an Orbi 970 router (10G WAN). Inside wiring is Cat 6, and both modem and router are current-firmware and recently power-cycled.  

Over the past two weeks or so I have seen time-of-day download slowdowns that weren’t present before. Early mornings are normal (≈2.30–2.35 Gbps), but evenings, and frequently afternoons, speeds fall to ≈1.70–1.90 Gbps whether I test from the router or from a directly wired MacBook Pro—so results track across devices. Upload remains perfect (~350-360 Mbps, over provisioned).  Note, the speed tests always seem to start high - 2300+, maybe even close to 2400 Mbps, then slowly fall to the 1700-1900 range. This seems to indicate to me that provisioning is correct, in that speeds start in the correct range.

Comcast support did assure me the boot file provisioning is correct for my service tier (2100/300).

My RF looks clean and stable: 
• Downstream QAM: +3.5 to +6.2 dBmV, SNR 43.5–44.5 dB (32 channels) 
• Downstream OFDM: Power ~+6.2 dBmV, SNR ~43.4 dB, 0 uncorrectables over 14 days 
• Upstream QAM: 43.3–44.0 dBmV (4 channels) 
• Upstream OFDMA: 38.8 dBmV

• Logs: <5 T3/day, 0 T4, 0 CM-STATUS. PMA profile changes are modest and typically settle on profiles 12+13 (highest throughput).  

Because RF and LAN look healthy and the slowdown is evening-specific, this seems like service-group/CMTS congestion or related capacity constraints. Could you please: 

1. Check node/service-group utilization and packet loss, especially during ~4:30–9:30 pm ET 9/14 since I included speed tests for that time period below; 

2. Review CMTS OFDM profile distribution for my modem and any PMA trims in the same windows (PLC/MAP SNR, profile time in 12/13 vs lower); 

3. Verify my modem’s config file matches the 2100/300 tier with expected over-provisioning and that there’s no unintended shaping;

4. Confirm CCAP/backhaul for this service group isn’t saturated or alarmed; and 

5. Run a PNM check for intermittent ingress/CPE leg issues that appear only under load.  

For reference, here are sample results (all wired; MBP=MacBook Pro): 

• September 14 –

4:33 pm: 1.98 Gbps (MBP) 
6:45 pm 1.84 (MBP) 
8:12 pm: 2.14 (router)  
8:18 pm 1.83 (router) 
8:21 pm 1.71 (router) 
8:34 pm 1.75 (router)  
9:26 pm 1.72 (MBP) 

1:21 pm: 2.30 Gbps

• September 15 –

5:48–5:50 am: 2.17–2.35 Gbps (router; 3 tests) 
6:44 am: 2.30 Gbps (MacBook Pro)  

These tests were all to the local Comcast site, less than 1 mile away.  

If it helps, I can provide additional timestamped tests to multiple third-party servers to rule out test-server load. Please let me know if you need MAC address, CMTS, or service-group IDs to correlate.  Thanks for taking a look—I appreciate any next steps to mitigate congestion (e.g., re-segmentation, capacity augments) if that’s what you see.