What causes my 2009 Chrysler 300 to stall when starting and also looses pressure

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When turn the switch to start, I let go and it continue to try and start and I'm not touching it. Also when driving it will looses pressure even when you press the gas pedal to the floor. After letting up off the gas the car gradually picks up speed. At times when it looses pressure and you let up off the gas pedal the car will shuddered.