Shuts off while driving

I've had this car for about a month and it hadn't started to give me problems until recently. I would drive to work and as soon as I hit 55 MPH the car would jerk and smooth itself out. I thought nothing of it until on my way back home. The car jerked while I was going 30 MPH and just turned off. All the lights on the dash board came on and I had to direct the car to the nearest mechanic. The thing is, as soon as I park it and turn it back on, it works. Like nothing happened.
At the mechanic they've replaced the crankshaft sensor and I even had them do a fuel system cleaning, but the car still turns off while driving. I've noticed that if I drive it for too long, it starts to shut off. But when I go to the store or any place where I don't go too fast, it drives normal. I've taken back to the mechanic and they don't know what the problem is. They say it could be a bad sensor. But they just font know which one. Not to mention there's no check engine light to warn me.