Car failed on high CO (.45) on idle and high revs.

Car runs fine apart from when warm and on idle it "missis a beat" every so often (5-15 seconds). Ticks over about 850rpm then suddenly just drops to about 750rpm and straight back up again.

The missing seems to throw the HC, CO, and O2 up and fails MOT. When revs are at 850 HCs and Lambda pass but C0 didn't go below 3 for long (0.25 best-0.65worse).

I have done full service and give it a thrashing with CAT cleaner in. I have changed Lambda & Water temp sensor from 4 other Polo's, also MAP sensor, Throtle body and CAT from one of them. Nothing seems to have changed.

Squirted soapy water every where trying to find air leak! No luck.

The clicking from the injectors (x4) seems to change about same time as the missing??

What more can I try?? Many Thanks.