Sounds like when you go into closed loop and the O2 sensors are "engaged" to manage your fuel maps, the ECU is not delivering the proper amount of fuel since the input is wrong. I won't dispute the injectors being potential, but given that the car was rarely driven at decent speeds when fully warmed up and in closed loop, I'd take the easiest first step and replace both O2 sensors and recheck. Guessing that at least one of your O2 sensors is badly coked and reading incorrectly.
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