「たまに」 means 'occasionally' or 'rarely'. It suggests a lower frequency than 「時々」. You use it for things that don't happen often, maybe once in a while or very infrequently, emphasizing infrequency.

非常(ひじょう)に少(すく)ない頻度(ひんど)で起(お)こることを表(あらわ)します。