1999 Buick Park Avenue Car has a lack of power when acceler

I have a 1999 Buick Park Ave Ultra. It has a lack of power when accelerating both from a stop and while trying to pass. The only code in the computer is P0453. I have replaced the plugs, wires, catalytic converter, and the sleeve between the air filter and throttle body. I have also checked for vaccuum leaks, found none, and cleaned the MAS. When I replaced the cat it did get better but not 100%. When I step on the gas it feels like the car is being held back and then suddenly it shoots me forward like there is no problem. When I step on the gas it also stutters as it tries to gain speed. Also since this problem started, the DIC shows 2 more gallons of gas used than what I am able to put in everytime. Any suggestions on where I go from here?