Inconsistent hesitation with acceleration

While taking this vehicle for a test drive, I was at a stop and making a turn, when I started to go the car would not move it was still running. After about three to five seconds it finally goes. The dealership assured me it checked out fine no issues. Well it happened may times after, it was brought back in and they are unable to diagnose the problem. They put a data recorder in for less than a week and of course nothing happens at that point. However it is doing it again inconsistent. This is a dangerous situation, I live on a very dangerous road and coming out is on a bad corner, the car did this hesitation thing as I was coming out and I nearly got hit. The dealership said that they have done all the testing and found nothing wrong? Hmm. Do you have any suggestions? I do not want to have to return the vehicle, but this is really becoming a safety issue. Do not know if this has anything to do with current issue, but also when I start my car it does not heat up until I drive for about three minutes or so. I had the car running for fifteen minutes yesterday and still had to scrape my windshield. Can that be one of the same issue? Sorry for the long message. Please help! Ann