DB:

In unplugging the EHA you have disabled the mixture feedback circuit, as noted above. Absent feedback, the mixture is controlled entirely mechanically by the airflow meter/fuel distributor package. That engine power returns to a near normal state suggests that there is an errant wiring connection between wires associated with the A/C compressor control unit (N6) and wires between either the ECU and the EHA, or between the O2 sensor and the ECU. Pulling the plug from the compressor, and repeating your driving test may tell you if the symptom is current related.