A "temper", to me, refers to a person's likelihood of getting angry in a situation. "He has a hot/volatile/even temper" tells you the likelihood of a person responding in anger to a given situation.
"Temperament" is more the overall "default" mood and emotional baseline of a person, in my opinion. "She has a morose temperament" means that she walks around in a state of moroseness without any outside influence causing that mood. "She has a sunny temperament" means that, with no outside effect or influence, she will tend to see things optimistically and approach situations in a cheerful mood.