My old tl wn821n v3 usb stick adapter started doing the same thing about thursday last week. I thought it was just getting old. It would max out at just under 4 mbs and then start to decrease til the Led light froze or went off completely. After a bit it would start up again. Any heavy downloading would make it do it again and again. It would work for  bit then drop down til it lost connection then reconnect.

I tried the drivers but there is only 2012 win 8 beta drivers for it. Tried uninstalling it and reinstalling it. Tried other usb ports. I use an old pc so i got a couple of bsods i thought must be linked to it and looked at the minidump analysis. Nothing i could understand. So in the end i tried a new fresh win 10 install on another drive. The new win 10 install wouldn't recognize it even with the drivers put in. It sees it as an Atheros adapter. It seems to working. Need to try it on a heavy download. Network speed is low though. Will try an ookla speed run test..

Well you have to fart with it but my old pc is a dinosaur. So getting back to 4 mbs is good enough. have to test it on a big fast download which means finding a downloader to download something at that speed.

Sorry didn't see the reply notification email. 

I use torrents for alsorts, you can monitor download speed at the torrent speed the downloader operates on. That's how you can see it looks like it's being throttled back as it reaches certains speeds at verying times. It's not static so it happens at random times and at random speeds. Of course that could be due to the torrent and seeds but that didn't matter as so much that once it started to happen it would throttle back slowly down to zero. Once zero was reached it would freeze ( Led light stuck on and would require to be pulled and reinserted. )  or  turn off completely. Most times that meant it had shut down then windows 10 would pick it back up again. 

It's on my old pc in the garage but close to where the router is in the house. The walls are thin and though it's not top signal strength it's constant at 70 to 90 signal strength. Since the last reply to you it started to throttle back when i put it under load. I had an old phone so i connected that and no throttle back issues or disconnection. All i can figure is that it's an old adapter that can't handle heavy loads any more. I somehow wrecked the atheros driver for it in my win 10 and it wouldn't recognize it anymore but would still pick it up, so i was able to connect via just automatic network profile set to private. Still got throttle back and disconnection issues.

I was using win 7 til 2 weeks ago then decided to switch back to win 10. I should never have made the jump. Win 10 1903 wrecked my constant and has given me nothing but problems. Once the adapter started acting like this then switching back to win 7 copy didn't matter. I have usb 3 issues so thought it was possibly a consequence of that but i don't think so. I did come across a forum where they said to reload the motherboard drivers to help with the usb 3 problems. It didn't matter as it did the same on the other usb ports anyway.

I should've mentioned i had a mini usb stick tp link adapter on another pc in the house and that stopped working a couple of years ago. 

The drivers available for my adapter tl wn821n v3 are beta for win 8. Theres nothing else for my adapter.

@azeraph I have a TL 8200ND and i have the same problem, constantly drops, with the adapter drivers and the windows recommended drivers ,idk what to do. 

@EWS Hi if you follow the instructions I've attached below you should be good to go.

Ok, so with a TL-WN821N v4 with Windows 10 version 1909, I've done what was explained above, asking to Windows to stop installing drivers by itself, trying to uninstall the driver using the Device Manager, disconnecting the TP LINK, ect. and it didn't work because the last Windows versions don't allow you to unistall drivers using the Device Manager. So I went directly to delete the driver in the system32 using right clic on your device on the Device Manager, Properties, Drivers and Details on drivers. I found two drivers that were used for this device the C:\Windows\System32\drivers\vwifibus.sys from Windows and the C:\Windows\System32\drivers\rtwlanu_oldIC.sys from Realtek. Modifing my access in order to delete the last driver correctly (modified the owner : "TrustInstaller" to "Yourself" and change the permissions of 'Yourself' on this file) (make sure you keep a copy on your desktop or somewhere) I managed to delete the Realtek driver in order to force install the TP-Link driver for Windows 8.1. and now, finally it's working !

To summarize, Windows install drivers for your Windows 7 Wifi USB Key and don't allow you to reimplace this RealTek driver easily passing by the Device Manager...

(Sorry for the English, I'm French).

Hope it will help someone and thank you Moboy for the idea and your tutorial !