New Customer - Widely Variable Download Speed - Time Warner Internet

This question is about "New Customer - Widely Variable Download Speed", with Time Warner Cable internet and apps. Had Internet only Spectrum service installed last weekend. Went on chat to report slow download on speed tests of around 5 Mbps. They had me reset modem, and did some things on their end. Asked what plan I had, they said 100/10. After first chat, speed increased to a steady 125 Mbps! Was able to dowload a 1.5Gb file in about 14 minutes. All good. Next morning, speed back down worse to 2-3 Mbps. Chat had me reset modem again, back up to around 100 Mbps. During all of this download speed variation, upload was rock steady around 10 Mbps. This weekend dl speed varies from 10-50 Mbps but mostly around 20-30 Mbps.
Dallas, TX 75219, 6 unit condo (2 are empty and one is ATT) so there are 3 of us on TWC.
I bought my own equipment which is Arris SBG6900-AC
Just did speed test on TWC site: 11.2 down / 11.9 up, Latency 22ms, Jitter 7ms
Cable is buried from pole at street to a box between an adjacent apartment building, this is what it looks like under the plastic cover, one coax wire (the black one at upper left) runs from this box unburied to our building ( about 10 feet) and into the garage.
In the garage distribution box, it looks like that one coax has a 3 way splitter for those of us on TWC
Some screen shots from modem:
What could be causing these wild swings in download while maintaining a perfectly steady upload? I am testing with wireless disabled and Ethernet straight to the computer, no programs running in background. Building was built in 1983, in good shape, and no reason to believe there would be problems inthe internal wiring that I know of.

This topic covered New Customer - Widely Variable Download Speed, and TWC cable internet service.