Kia reports that some 2001-06 Optima models with 2.5-liter or 2.7-liter engines along with 2005-07 Sportage SUVs with 2.7-liter engines may hesitate under moderate throttle.
According to the car maker, the drivability problem may be accompanied by an illuminated Check Engine light, with trouble code P0123 stored in the PCM. A glitch in the throttle position sensor signal is a likely cause of all the problems. Replacing the sensor with an improved design, part #35170-37100FFF, should eliminate further issues.