I have a Dell XPS 13 Developer Edition (L322x) with the Atheros Killer wifi card installed. It's a lovely laptop, but the wifi was hanging for 30 seconds or so, or dropping and reconnecting frequently.
After going down several blind alleys on various forums, I found the simple fix is to update the kernel. Open up a terminal (press CTRL+ALT+T) and type in:
You'll be asked for your password.
Once the install has finished, reboot your laptop and all will be good.
After months of attempting to fix this issue, installing this backport worked for me using the provided instructions.
The new drivers should then be in use, but note you will need to do the steps 4-7 again if a Linux kernel update comes as a security update or otherwise.