Is this fault intermittant ?
Running on two cylinders when hot would suggest
packs, ht leads or plugs have a fault..Have these been checked by the dealer ?
If you have a friend with a little car knowledge ask them to check these parts for you. Look for dirty or damage plugs like this..
A lot of the 1.6 engines seem to struggle running the aircon whilst idling or sitting at the traffic lights. Do you find this ?