Check the throttle valve switch. The connector is on the intake manifold at the front, slightly to the left, and has three pins. This switch tells the control unit when the throttle plate is at idle, or at full load. I don't remember the color of the wires, but it's easy to test with an ohmmeter. A quick way to check this is to determine which pins are for the idle signal. Just bridge the appropriate wires and see if it makes a difference in idle speed/quality. What you're essentially doing is fooling the control unit. Make sure you determine the correct wires before you do this. The great thing about this test is that it's quick, free, and CONCLUSIVE.