Maybe just a suggestion, but has the customer tried running the game under a newly created user account?
In a new user account you are sure that all OS settings are default. If it runs under that account perfectly it may be something that starts up automaticly after logging into the user account that is in conflict or a setting somewhere in system preferences that is not default anymore.
In a new user account you are sure that all OS settings are default. If it runs under that account perfectly it may be something that starts up automaticly after logging into the user account that is in conflict or a setting somewhere in system preferences that is not default anymore.