Volvo Repair: Rough idle when cold, vacuum leak, damp weather

Question
I have a 1990 Vovo 740 turbo with 280,000K on it. When I start it up it idles very roughly, almost stalling on occasion. After a few minutes it smooths out a bit but still isn't exactly silky. When I drive off and stop at a traffic light it idles roughly again and I have to slip it into neutral because I'm afraid the engine might fall of it's mounts from the roughness. It seems to get better after about 30 minutes but still isn't great. It's most rough when it idles at 750 and sometimes when at stop it idles at about 950. There is no rhyme or reason as to when it idles at 750 or 950. When it idles at a stop at 950 and when I put the gear in Park prior to turning the engine off the idle speed goes up to 1500 RPM's.The problem seems to be worse in damp weather. The temp at this time of the year in Southern Ontario ranges from about 15 to 35 degrees F. The car runs fine on the highway ar speed. The throttle body has been cleaned and the IAC valve has allegedly been checked and lubricated. Also there is a hissing noise that seems to come from underneath the car. Could that be a fuel pump issue. Any guidance would be appreciated.Thank you.

Answer
It sounds like a mixture issue.  If the car is running too rich or too lean then the engine idle quality would suffer.  The hissing sound you are hearing may be a vacuum leak, which could be the source of the problem.  It also may be an exhaust leak (it would have to be big to hiss though).  That could throw off the O2 sensor readings and then the computer would give the wrong amount of fuel.  Is there a check engine light?  Anything that affects fuel delivery/ air consumption should be investigated.  Vacuum leak is what i think is most likely the problem though.  It would have a poor idle but would run normal otherwise.

James