Stalling when engine at running temperature

I have been encountering stalling when coming to a stand still, and a slight hunting of the engine on stopping, the car runs smooth and with normal pulling power when driving, (car at running temperature).
So I took it to my local BMW dealership who did a full test on the system and could not find a fault. I decided to change the 8 spark plugs ( like for like) and on 2 of them found oil' on and around the plug threads near to the sparking area, the 6 other plugs where dry and the colour u would expect ( light grey)
So I'm thinking maybe this could be a cause for the stalling, I would be most grateful if you could you please advise on my comments or any other suggestions you may have. Once again thank you for your help. John Roberts