and check the engine temperature. The ECT sensors in these cars a sometimes known to go bad, and lie to the ecm and tell it the engine is cold when in all actuality it isnt. Which might explain why it runs good when cold is because the air/fuel ratio is close to optimal given the conditions. When the motor warms up, less fuel is needed. Keep in mind that there may be two temp sensors on this car, one for the guage and other for the ecm. You could just have it hooked up to a scan tool after driving it for a while and see what the temp is at, if its abnormally low, its safe to say the ECT sensor for the ECM is at fault. How does it start when warm? Hard to start? Smell Gas? Maybe get a puff of black smoke from the exhaust when starting warm?