Sounds like you are pulling your hair out, but seriously, you don't mention what kind of car, what year it is and what year was the northstar you put in, if it was different? Now, also, pre 1996 is different from after 1996, OBD-1 to OBD-11. Knowing this will narrow down the problem. Does it rev in gear, or just in idle? What have you already changed to correct the problem? eg.. Coolant Temp sensor, idle speed control motor, Throttleposition sensor? ECM, Crankshaft Position sensors. Mass airflow sensor, or EGR. Or it could be something as simple as something not connected, the groundstraps on the block not connected, but more info is needed..