Recurring loss of upload bandwidth during specific time range at night

I am currently subscribed at the 1200/(35) Mbps plan and live in a taller apartment building. Since moving in last month, I have been having consistent nightly internet issues from approximately 6:00pm to 5:00am - during these night hours, upload speeds will degrade to <0.3 Mbps, low enough to effectively time out all outbound requests. Download speeds oddly are unaffected, and when a Xfinity/Fast.com/SpeedTest.net/Google speed test does execute, I see 300Mbps+ download speeds (and <0.3Mbps upload). Despite this, without upload bandwidth the internet is unusable. I have worked with the two Xfinity technicians so far in attempting to diagnose the issue but that has been unfruitful.

I have tested different modems (S33, xFi Gateway, SB8200) different computers, different Ethernet cables, different power outlets, and technicians have checked signal integrity between my building floor's tap and the coaxial cable in the apartment during regular business hours - all seemingly clean with no signs of noise or ingress. They have mentioned that power levels were high at the tap, but should level off once in my apartment. I am not sure if other residents on the same my floor's tap are experiencing similar issues at night. What is frustrating is that during the day (5:00am-6:00pm) internet speeds are nominal (1300Mbps down, although upload always only caps at 20Mbps). Only after these hours does it degrade, and technicians are unavailable outside of business hours, so they are unable to see the issue firsthand.

Is it possible that some damage has occurred to the line into my apartment, and the temperature fluctuation of day and night exacerbates the issue - during the evening my upstream power levels are about +1 to +2dBmV from what is shown below. Another suspicion I have is that an automatic power source of some sort (timed street/building lamp?) is feeding noise into my unit’s line during these hours. 

I appreciate having techs sent over to inspect but I feel like their time is wasted when they do not see the issue firsthand. Any assistance or pointers are greatly appreciated. Kind of getting tired of not having internet at night :)

Edit: The technicians have also said their "network guys" don't see any issues at night.

Arris S33

I see log Event Type Codes 16, 20, 22, and 24 for channel 48, but never any T3/T4 timeouts.