Mitsubishi Repair: I OWN A 1994 ECLIPSE GS DOHC..., stepper motor, air leaks

Question
I OWN A 1994 ECLIPSE GS DOHC 16 VALVE AND AFTER RUNNING THE CAR AT 60 MPH FOR A PERIODE OF TIME THE CAR WHEN PLACED IN PARK WILL IDLE ROUGH WITH THE TACH BOUNCING BETWEEN 10,000 AND 15,000 RPM. THERE IS ALSO A JOLT WHILE DRIVING AS YOU SLOW THE VEHICLE DOWN TO STOP AND THE IDLE AT A TRAFFIC LIGHT IS BETWEEN 10000 AND 15000 RPM. THIS CONDITION JUST STARTED LAST WEEK WHEN IT WAS COLD HERE IN NEW YORK AND AS THE WEATHER MODIFIED IT SEEMED TO HAVE ABATED SOMEWHAT BUT WITH THE WEATHER COOLING DOWN AGAIN IT HAS REAPPEARED.  

Answer
John,
This sounds like a problem with your Idle Speed Control (ISC) stepper motor.  This component controls the amount of air that bypasses the throttle plate when the it is closed (i.e. when you take your foot off the gas peddle).  The ISC will regulate your idle speed, and if it is malfunctioning, it make a best guess as to where the idle speed should be and cause your idle speed to bounce around.  It is also affected by engine and air temperature.  But other items to check are your mass air sensors and check for any air leaks in your intake system.
Good luck!