HELP!!! mk4 golf gti trouble

[wadi]

hi there let me just say what a great site this is!!!!! right down to business, i have a golf gti 2001 y reg 2.0 8v 80,000mls the engine managment light has come on and i have read the fault code which is saying, 16804 catalyst system bank 1 efficiency below threshold, po420-35-10 intermittant, i was using vag com software to read the fault, i cleared the code and the car was ok for 1 week then the light came back on and the same fault code came up, do i need a new cat or a lambda sensor? or is there any common faults on these cars? air leak etc etc?
any help would be great thanks!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!:biglaugh :