Not sure that this is such a good steer? The manuals say not to alter this and clearly a fault has developed in the OP's car and adjusting the idle speed would just be a way of masking the problem. I'd start by cleaning the idle air control.
Thanks for the ideas guys.
It's the 3.8 V6 motor, it's been serviced regularly, plugs replaced just before the silly season started, and it makes no difference whether the tank is full or not it is doing it all the time, have even tried injector cleaner without success.
Thanks again and stay safe
