u/East-Sign-3663

TL;DR: Persistent peak-hour upstream jitter (avg 55 ms → p99 185 ms → max 618 ms to multiple unrelated public destinations) that starts at Spectrum's first hop (172.29.152.65). Two field techs have already verified my premise wiring, drop, and modem are clean. Modem reboots don't fix it. Asking for a CMTS/node-level review, not another dispatch.

Setup

• Service area: SoCal (full address in DM)
• Plan: Internet Ultra tier (~1 Gbps / 35 Mbps, DOCSIS 3.1)
• Modem: Spectrum-rented, installed 2026-05-06 (replaced my Netgear CM3000 after a tech diagnosed a stuck-high upstream Tx power and only 2 of 8 US channels locking — that fix held, signal currently looks fine)
• Router: Spectrum-provided Sagemcom
• PC: Wired 2.5 GbE direct to router, WiFi disabled, no powerline/MoCA in the path
• CGNAT: Yes, first public hop is 172.29.x.x
• Public IP at time of testing: 98.149.235.x (full IP in DM)

What's already been verified clean

• Field visit #1 (2026-05-06): Tech replaced the modem and reterminated the drop after diagnosing stuck-high upstream Tx and US channel lock issues. T3/T4 timeouts went away.
• Field visit #2 (more recent): Tech reverified signal levels at the modem and the ground block, confirmed clean. Said premise side looks good.
• Self-checks I've done:
  • Modem reboot — no change
  • Router reboot — no change
  • Tested with router bypassed (PC straight to modem) — same behavior
  • cake/SQM on PC at 36 Mbit upload, ack-filter, docsis overhead — Waveform Bufferbloat test grade A (so no local bufferbloat)
  • Disabled my NordVPN to compare paths — issue is actually worse without VPN, because the VPN was bypassing Spectrum's congested route

The data

Local LAN to my router (300 consecutive ICMP, 0.2 s interval)

Metric	Value
min	0.4 ms
avg	0.8 ms
p99	1.1 ms
max	2.3 ms
stddev	0.1 ms
loss	0%

Perfectly clean — proves my LAN, NIC, switch, and the cable on my side of the demarc are fine.

Game server (Google Cloud, same sample window, ICMP)

Metric	Value
min	34.5 ms
avg	55 ms
p50	43.6 ms
p99	185 ms
max	434 ms
stddev	38 ms
loss	0%

Cloudflare 1.1.1.1 (same window, completely different ASN/path)

Metric	Value
min	11.5 ms
p99	180 ms
max	547 ms
stddev	65 ms
loss	0%

Identical jitter pattern to two unrelated networks at the same time — rules out any single destination, peering, or remote endpoint being the cause.

mtr trace showing exactly where the jitter is introduced

hop  IP                 loss   best   avg     worst   StDev
 1   192.168.1.1        0%      0.6    0.8      0.9    0.1   ← my router (perfect)
 2   172.29.152.65      0%      5.6   27.4    163.2   35.1   ← Spectrum CGNAT — jitter STARTS here
 3   76.167.25.16       0%      6.7   21.8     94.4   18.8
 4   76.167.25.20       0%      9.5   32.9    209.3   41.7
 5   76.167.28.150      0%     10.2   27.9    122.1   30.9
 6   72.129.7.244       0%     12.4   28.9    149.9   31.0
 7   66.109.6.92        0%      9.2   44.0    538.6   97.9
 8   66.109.6.6         0%     35.2   67.5    462.3   78.4
 9   66.109.9.45        0%     33.9   62.9    365.2   61.0
10   34.4.108.40        0%     35.2   55.0    267.2   42.9

• Hop 1 (my router): stddev 0.1 ms.
• Hop 2 (your first router): stddev 35 ms.
• That's a 350× jump in jitter at the first device past my modem, with zero packet loss and no bandwidth saturation on my side (peak utilization during the tests was ~80 KB/s — nowhere near my plan's limit).