super slow speeds 0645-1845 daily

WELL it looks like this is finally resolved. I'll elaborate in hopes that it saves someone else a headache later on.

Tech came out, still only getting 10 mbps downstream. He runs a ton of tests outside, everything looks fine. Finally he brings his laptop in, and connects it directly to my modem, and gets full proper speed via ethernet. Connects to a LAN port on my router (TP Link Deco XE75) and gets the crappy speed. SO - the problem is isolated to my router. Which doesn't explain the 0645-1845 timing thing detailed in my first post, but clearly whatever is going on is a router issue. Right?

He leaves, I order a new router, after first attempting to install a previous version of the firmware for the router, no luck. But - full speed suddenly starts happening that evening, a few hours after he left, during the timeframe it would have been throttled previously. Weird. So I do nothing and it's fine, for about a week. Then I start getting 100 mbps instead of 1 gig (I had upgraded to 1g from 500 mbps in the middle of this). Reboot the router, back to full speed. A few hours later, back down to 100 mbps. Now rebooting or power cycling it do nothing, still slow. I decide it's time to replace the flaky router for sure.

Then I'm going through the router's app and see a buried message that there's an issue with my WAN port speed, and that perhaps the patch cable can't support 1g speeds. When the tech was here we actually changed that cable to no effect. I had a brand new one lying around so I swapped it. Boom, full speed again, no more error message about the WAN port. The new patch cable is CAT6. The old one was 5E. Hmm. CAT5 is only certified to 100 mbps. 5E is not an "official designation" but is supposed to be better. CAT6 is rated to handle 10 gbps at < 121 feet. Ah, massive improvement.

This seems to be the final solution, and my presumption is that it was a patch cable issue the entire time. The only weird thing about that theory is why was it working fine for the same 12 hours every day, then throttling down to 1/10th speed the other 12? That remains a mystery. All I care is that all is well now.

Thanks to everyone who responded, and thanks to the excellent Cox peeps that responded to my email and helped me get the tech scheduled, which is a NIGHTMARE to attempt to do via phone support. And thanks to the tech himself, he was awesome.