Check the TPS for adjustment as another respondent posted. Also check it with a continuity meter. You may want to get a wiring diagram and check it from the ECU connector.

The electrical connectors for the TPS and the Air Control Valve motor are identical. Having them swapped could be your problem. Make sure you have the right connectors on the TPS and IAC. TPS wires are orange and brown, ACV wires are green, yellow/rec, and gray/red.

Once you have verified the wiring connectors are correct, check the Air Control Valve for electrical continuity from the ECU connector. You should have about 20 ohms resistance from terminals 10 and 23 to terminal 18. The Air Control Valve is supplied with 12 volts at its wiring connector from terminal 18 of the ECU. It is alternately grounded at terminals 10 and 23 to position the motor.

Since you have reported that the idle speed drops to as low as 500 RPM, you need to set the basic idle speed. This is covered in the 700/900 FAQ, but I will repeat it here. Find the two wire connector on the right inner fender well. It will have two wires, a red/white and a green/white. With the engine hot, ground the red/white wire. This will close the Air Control Valve. If it doesn't close, you have a problem with it or the wiring. Once the Air Control Valve closes, then adjust the idle speed to 725 plus/minus 25 RPM. The green/white wire is used for setting the idle mixture, which is also covered in the 700/900 FAQ.

Let me know how you do.