So a few days ago my vacuum boost level has been sitting around -15hg and while it does this there is faint black smoke coming out of the exhaust and it idles really rough. After driving around a bit it usually goes away, vacuum and idle go back to normal. But after the car cools down and i start it up again it goes back to the rough idle. Has anyone experienced this, any help would be much appreciated

 

Its a 2005 ss/sc