what would cause engine to idle rough,cuts off but will crank back

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replaced bad mass a/f sensor,cleared codes.got running & no codes returned.low & rough idle,cuts off.ck fuel pres. & found it was too low @ injectors.replaced pump,still have idle problem.runs great above idle.also has new s.plugs,air filter,cleaned intake.drives good above idle,high speed no problem.still not codes.a BIG thanks for any help,jimmy.email jhstatham@gmail.com