Hi, Iím after some help please.

I have a 2002 Golf MK4 2.0 engine code AZJ.
I have check engine light on and the code is P0420 (Catalyst system efficiency below threshold Bank1)
looking through forums this seems to Be quite common but no one ever seems to get to the bottom.

So far itís had
-New spark plugs
-New HT leads
-New coil pack
-New front and rear OE Bosch lambda sensors
-Cat clean

ive also checked for air leaks best I can and taped up a vacuum pipe going to the brake servo that was slightly split to see if this makes any difference. However the check engine light is coming straight back on after clearing it after about 40 miles. It looks like itís had a new Cat converter at some point possibly (however thereís no signs of any exhaust leaks except from very very slightly at the manifold. When I changed the 02 sensor the CEL stayed off for about 140 miles.

Symptoms ive got
-CEL coming on 40-100 miles after clearing
-The revs sometimes dip when idling
-funny/ rich smell from exhaust sometimes
But surely the CAT converter canít be bad as it recently passed the emissions on its MOT test just fine.

Please let me know youíre ideas as im almost at a loss with this but want to get it fixed as I like the car but also donít want to just keep throwing parts and money at it.

Thanks guys