This Mitsubishi motor runs fine for an indefinite period then begins surging to such an extent as to be undriveable. A Dodge mechanic analyzed it and assured me it was the Idle speed control (or Idle Air Control), which I replaced, and the problem remains the same. Someone else suggests the EMC or engine control module.
If there are no vacuum leaks, and the minimum idle screw has never been touched then you should try cleaning out the throttle bore. Use a little carb spray and a rag to clean where the air intake hooks up to the engine, clean all around the throttle plate