2004 Mazda 3 Dont know if electrical or Engine

Electrical problem
2004 Mazda 3 4 cyl Automatic

Recently my car has been shutting off the whole car meaning radio and all lights shut off than after it shuts off it is very hard to move my steering wheel but turns back on. The part that sucks it does it when ever. I did go to the dealership hooked up to test my engine and it came back all good. They test drove my car and nothing happened but right after I left it shut off and did the same thing again. I paid over 500 dollars for what I do not know.