1998 chevy malibu dies while driving down road

I had the engine changed in mu 98 malibu. Now when I drive down the road the car will just die. If I can get it started again the engine will start cutting out at about 400rpm and then it dies again. This will happen about five or six times and then the car won't start at all and must be towed. It will still turn over though. I've had it back to the shop three times and keep getting the run around.