HELP!, been working on this for days.

My internet connection is squirrely, mostly working but I'm having issues with certain apps.

My internet plan is 500/500 mbs via fiber optic, using ethernet, and has been running fine. All of a sudden it only does 100 upload, and there are anomalies all over the place hinting that something is amiss.

It's not either the ONT or the router. That has been verified by ISP tech support, and the fact that a laptop plugged into the same network switch (same port/same cable) that the questionable computer is plugged into gets 500/500. I tried other ports and cables too.

Runtime anomalies:

-Thunderbird (email client) twirls and twirls, and never downloads the latest email. I can do this fine on a laptop using the identical software (Thunderbird portable) via wi-fi. I copied the entire Thunderbird portable folder to the questionable computer, so they are identical.
-WordPress, when working on a test site on the internet, cannot either delete a plug-in or download/install a backup from my local WordPress environment. I can successfully do the same things working on another computer.
-Oddly, web browsing is fine.

What Iíve done:

Cleaned out the registry endlessly.

I've replaced the ethernet card and deleted and reinstalled the network driver. I've also disabled the ethernet connection and connected via a 2.4gHz USB wi-fi, and it tests out at 36/25. However, my cell-phone gets 46/47 on 2.4gHz and 226/286 on 5gHz. Don't know if this means anything. I really need a USB 5 GHz wi-fi.

I've also tried different cables and different ethernet ports on the network switch which works fine with a laptop.

I'm going to try next reinstalling windows on a temp drive just so I can see if it's my Windows environment.

Sorry to be long-winded but I want to illustrate that it's got to be the computer, and avoid well-meaning comments like "try another wi-fi channel".

Processor Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz 4.01 GHz
Installed RAM 64.0 GB
Windows 10 Pro
Version 22H2
OS build 19045.2788
Experience Windows Feature Experience Pack 120.2212.4190.0

Other hardware:
NVIDEA GeForce GTX 950 graphic adapter
SSD boot device, 2 internal SATA hard drives
Wacom tablet
Firewire 1394 PCIe card
wireless keyboard/mouse

-- Thanks for any suggestions!