Shocking eco sport engine

Car due for second service. Only 18000 km on the clock and my enitre engine needed to be replaced. Took a week to replace they gave me a courtesy car which was lovely and appreciate that very much. Got my car back on Saturday hit the highway on Sunday for a family braai and, feels like the engine was back firing, was cutting out and jumping strange clicking sounds comming from new engine and when I was in 5TH gear the car cut completely and I could not excelerate!! Seriously felt like my life was in danger driving the car. How is this even possible. Totally unacceptable.