Episodically slow internet with brief outages - Time Warner Internet

This question is about "Episodically slow internet with brief outages", with Time Warner Cable internet and apps. West Hollywood California
Service: 100 Mbps Internet plus TV
Modem-Router Netgear C7000-100NAS
2 splitters between tap and modem, the first with the modem line on the 1.5dB side, the second symmetrical
Downstream power -4.9 to -6 dBmV, SNR 40.5 to 42.5 dB, Upstream power ~46.8dBmV

For some time, downstream internet speed (wired) wanders around between 25 and 120 Mbps with momentary disconnects (which show up in TWC's logs) which irritatingly restart streaming shows and cut off Skype sessions. Otherwise, TV reception seems not affected by whatever's wrong. Upstream internet speed is almost always within spec (over 10Mbps). I have VoIP phones and get momentary garbles

Action so far: TWC replaced the drop (eaten by rodents at the tap) and grounding barrel, and remade all cable terminations in the house. No improvement. I rented an Arris SB6183 from TWC to rule out modem problems, used it with a known-good Netgear 4500 - no improvement, which in my mind rules out problems with my Netgear C7000-100NAS modem/router.

TWC reckons it's a signal quality issue (I'm guessing that means not related to my equipment) and a tech is coming out on Friday. However, the last tech was out of ideas - can anyone help with what I should ask him or her (esp. MsRaye, your take-no-prisoners answers are hilarious!)

This topic covered Episodically slow internet with brief outages, and TWC cable internet service.