Car runs rough when cold but runs once better when operating temperature.

I have 2000 Jaguar S-Type 6-cylinders. It runs rough when cold but runs once
better when operating temperature is reached. My mechanic confirmed the
plugs were fine but replaced the solenoid. It was still doing the same thing. He
did say it might be air leaking along the line but yet to find it. I guess there is
hissing noise but unable to locate where it is coming from.