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If you start your mac in safe m ode, does it function better? And if so, can you get online in safe mode?
To start your mac in safe mode, start with the mac turned off. Press power then *immediately* press and hold the left shift key on the keyboard. Keep holding it till you hear the startup chime, then release. That should start you in safe mode.
Once in safe mode, let me know if you can get online or not.