1996 Dodge Intrepid Rough Idle and Stalling at stop

My 1996 Dodge Intrepid has developed a condition where it idle's rough (rpm drops below 500 at times, engine surges up to 1500 at times). This happens whether in park, drive, neutral, etc. Another symptom is if the accelerator is used to increase the rpms up to 1000+ or so, letting off of the accelerator causes the rpms to drop all the way down and the engine dies. Once the car is moving it appears to run fine with no audible missing or other problem until it comes to a stop where it stalls. Restarting can be hard, but it always restarts successfully. The only things I notice is an audible "hissing" sound under the hood at idle which could possibly indicate a vacuum leak, and the exhaust smells like the engine is running rich Any comments/advice would be appreciated. I have this car into a non-dealer mechanic a couple of times and the problem has never been correctly diagnosed.