Well you have established the car is not going into a closed loop operation.
First and foremost establish if the O2 sensor is functioning. Pull the passenger footwell carpet back and you'll find the O2 sensor plug in there. Use a DMM on it when the car is warm and see if it reads any voltage. 0 is lean, 0.9v is rich. You should see it fluctuate around ~0.45v. If there is no voltage -- replace it. I read you did replace it, but did you replace with a new one? Either way you should check for operation so you can definitely rule it off.
Here is a link:
http://www.landiss.com/mixture.htm
#5 at the very bottom.
Bosch MB sensor costs ~$125, but you can splice in the same Bosch O2 sensor from a 1990 Mustang GT for ~$40. You pay quite the premium for the 'plug'!