CAT warning
 

A few weeks ago I had my Cat warning come on along with the missing cylinder/shakes. I did the test for a duff ignition amplifier (disconnect each one and see if it runs. Good fun with 3 cylinders!!) and one had gone so changed that. I've just had the Cat warning and judders come on again so tested the amplifiers again and the car ran exactly the same with each amplifier disconnected?
Is there anything else that I should look at? I'm hoping to have it on VagCom tonight or tomorrow.

Plugs?