Vehicle Shudder problems of the 2010 Mazda Tribute

While driving on freeway during peak traffic (stop and go) all of a sudden my car would not go when pressing on gas pedal and wrench light came on. Because I was on a slight decline my car very slowly crossed two lanes of oncoming traffic and finally made it on the side of the freeway. Car started to shake really bad before turning off ignition. Got car towed to dealer and they couldn't fine anything wrong after keeping car for one week. Picked up car on 12/12/13 and same exact problem happen again on 1/8/14, had car towed to dealer and they couldn't find anything wrong after keeping car for one week. Picked up car on 1/18/14. They said they can't do anything unless the wrench comes out. My concern is when I'm driving on the freeway and cars are going 45 - 60 mph, I'll get hit from the back and probably get badly injured or may even die!.