For some time now my 97 altima (232000miles) has been acting up. On a cold day when I start it up, it run good for a minute and then it idles really rough. It revs to about 2000rpms and then it drop to under 500rpms; it does this until it stalls. When I try to start it, it turns over good but does not start right away. When I get it started it putts and sputters and blows out black smoke. There is also alot of exhaust fumes. When driving it goes fine,but when we come to a stop the idle drops like it wants to stall and sometimes it does. At time it bucks while accelerating and feels like its choking or like its not getting enough fuel. We had the plugs, distributor with new caps and rotors, knock and crank sensor changed. The mechanic checked the fuel filter, fuel pressure, MAF sensor and the idle control, but it still acts up. I checked the vacuum lines running to the EGR and saw one of them has a small crack. Could this be the cause of all this or could it be something else? There is also a small hissing sound coming from the back of the motor by the intake manifold. Could this be another cause?