What is the fuel pressure doing during the fifteen minutes? Put a pressure gauge on the rail and watch the pressure. Could be as the regulator gets heat soaked it is dropping pressure. and the ECU is trying to compensate by adding fuel. Also check the MAF readings, at 2500 rpm no load should be about 9 ~ 12 grams/second and then on the road WOT should go up to about 90grams/sec.