Could be a corrupt preference file... Boot into safe mode and see if the behavior persists. Make a new user account and check there also. If either of those work fine you've eliminated hardware as source of trouble.
You could migrate a few files from the old account to the new and see if the problem comes back. Potential problems: corrupt fonts, or preferences from application files. If you go this route it would be best if you only migraged important documents and rebuilt all other files.
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