I drive a 2002 chevy impala (3.4) with 294,000 miles. The majority of these miles are interstate miles. Recently my cats engines has been sputtering at high speeds while going up hill. I've had new spark plugs and wires and fuel filter put on. When driving on flat land it's perfectly fine. It's just going up hill or slight inclines. But as soon as I push gas peddle way down it'll go just fine.

 

You might want to check your fuel pump's output pressure.