car shuts off after driving a while.changed the fuel filter..didn't help

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we just got the car.It wasn't taken care of.We took it to lakeside muffler and it was doing good till my daughter drove to Colorado. It kept shutting itself off. It would start again in about 20 min. Did this all the way there. Now it still does it even just driving to town. We changed the fuel filter but no change. we are thinking it is the fuel pump.