iMac wifi Blink

Here's a few low tech things to try.

• Perform a Time Machine backup and if possible a separate hard backup
• Uninstall any and all antivirus/cleanup apps and turn off VPN (if installed)
• Restart and see if things improve
• If no improvement, restart in safe mode and see if the problem resolves
• If yes this suggests that there is /are background tasks loading on a normal startup which are the cause
• Check your login items to see what tasks are loading and disable them one at a time followed by a normal startup. This will identify a rogue startup item
• A sledgehammer solution is to do a clean (virgin) install of MacOS followed by installing the latest version of apps one at a time with checking for problems before installing the next.

Try restarting your router?

Startup items are in:

System Preferences - Users & Groups - Highlight your account on the left - click the Login Items tab. It may look similar to: