98 Astro Van,when warm hesitating, stalling

I have a 1998 Chevy Astro. Runs great until it gets warm then it starts hesitating whenever you take off or turn a corner (want to make sure you have plenty of room to pull out). Once this problem starts for the day it will also stall when it idles at a light. ses light is on and reads code for the o2 sensor. did spark plugs and wires, iac valve, distributor cap,pcvalve, fuel filter, mass air flow sensor, thermostat, heating cooling sensor, cleaned o2 sensors and egr valve. all vacuum lines are fine. any ideas as to what is wrong?