electrical problems switches off engine by self

-When I drive my car the light engine start up then engine lost a power after a few seconds switches off by self. Then I have to turn off the key then turn on again and problem is gone.
After 100km-300km happen again, (same times happend very often).

I was at the mechanic electrician he scanned the engine by computer but didn't find what a problem is. That cast me $250.

Please for any answer.

Thank You Chris