Shuts off while driving

I bought the vehicle listed above three weeks ago, On Saturday while I was driving on the main road the vehicle switches itself off everything was lock luckily I was approaching a robot. I tried to start the car but it didn't start gauges were on and the was a security key display that was not going off after 15 minute the car started, I drove for about 10 minutes and it stopped again, I was so scared with my wife and my 5 months old baby as the car doesn't notice and uncontrollable when it stopped, it does that 4 times before I reached home. Yesterday I took it to a mechanic minutes from my place. He put his machine in the car but he couldn't diagnose the problem, it did the same thing 3 while the mechanic was test-driving it, I'm worried that next me I wont be so lucky when it does that on the road. Please advise how can I sort out this problem. Regard, Sydney