What will cause my car to shut off?

Well I just bought the car 2 wks ago so now im learning the car myself but it has 177 miles on it, so are u suggesting me to change spark plugs too? And I will have a mechanic to chk and see if it is indeed the fuel pump going, I hope thats it cuz the car run fines, just only if im going a consistant speed it just cuts off after going 55mph.. thats very scary driving on the highway when it shuts off like that