The idle/WOT switch is on the end of the throttle shaft under the fuel distributor. It is difficult to reach, but has a three lead pigtail (idle, WOT, and ground) that you can disconnect at the harness connector to test the switch for function. Also check the linkage microswitch for proper function,

If the Idle contact on the throttle switch or microswitch is not functioning properly or consistently, the ECU will not know that the car is at idle and will not send the proper commands to the ISV to maintain a stable idle speed.

Duke