WordWeb Online

1. Subject to sharply varying moods
   "a temperamental opera singer";
   - moody
    
2. Likely to perform unpredictably
   "a temperamental motor; sometimes it would start and sometimes it wouldn't"; "that beautiful but temperamental instrument the flute";
   - erratic
    
3. Relating to or caused by temperament
   "temperamental indifference to neatness"; "temperamental peculiarities"

See also: emotional, flakey [informal], flaky [informal], undependable, unreliable

Encyclopedia: Temperamental