Why do my low voltage exterior lights go out after only a few weeks?

My exterior low voltage lighting, installed by a trusted electrician, burn out after just a few weeks of usage. They are on each night from dusk to dawn but the first of them started burning out after just three weeks. Now three months after installation more than 1/2 of the 26 lights are out. They are under the eves of the house and therefore protected from the weather but I live in Phoenix where it is very hot.