Chevy tech will chime in on this... i'm sure...he has some really good information....
replacing the o2 sensor was a good guess, and a good place to start.
I would want to check fuel pressure and the coolant temp sensor for the ecm.....
fuel pressure is hard to check on the tbi engines. but the coolant temp sensor for the ecm is very easy to check.... check the resistance readings of the sensor and also the connector for the sensor, and the ground wires located on the thermostat housing.
link for checking coolant temp sensor
link for checking fuel pressure