Please read this whole message before doing anything.
Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode* and log in to the account with the problem. The instructions provided by Apple are as follows:
- Shut down your computer, wait 30 seconds, and then hold down the shift key while pressing the power button.
- When you see the gray Apple logo, release the shift key.
- If you are prompted to log in, type your password, and then hold down the shift key again as you click Log in.