Engine Problem :/

[Bigt0160]

I have a fiesta 1.6 zetec s 07 plate my car idels bit funny going from 750 rpm to 1000 rpm constantly at traffic lights I blip the throttle and it sorts it out any ideas ... Pluss my engine light is on code was bank 2 cat problem iv replaced the sensor but came back on after 400milres so checked the cat and the emmitions are fine I do have a performance exsorst .... Can anyone help .