You didn't say if the Mountaineer re-starts right away after it stalls. If so, you may have an IAC (idle air control) valve that is faulty. If it is a fuel problem, a fuel pressure gauge should be left on the vehicle while it is driven to see what happens when it dies. A fuel pressure regulator on the return line to the tank could be open and lowering the fuel pressure. Plugged cats will reduce acceleration and both top end speed and power, and often cause a hissing noise from under the vehicle.
Jim O'Neill - Chino Autotech Inc., Auto Club SoCal AAR