Engine misfire

Our car has an engine misfire in cylinder 3. The misfire only happens when the car is stopped with the engine idol. If sat at a set of traffic lights for too long it gets worse. We've had the engine checked. It's definitely piston 3. We've had all for coils replaced. All the spark plugs have been replaced. All of the injectors have been removed and cleaned. All of the wiring has been checked and is all in order. When using premium grade unleaded fuel it hardly ever happens. When using regular grade unleaded it happens a lot more.
We've been told that it might be the exhausust values but there is no black smoke from the exhaust and the engine doesn't stall.
Nobody seems to know the cause. The most recent pressure test gave a reading or 86 for piston 3 compared to 160 for the others.
Any ideas would be really appreciated.