check eng.light

i have a 93 trooper 3.2 sohc 165,500 on the motor.the check engine light comes on only after the engine is warmed up and only after it reaches 2200 rpm.it goes back off after it drops down below 2200 rpm.been doing this for over a year.engine runs fine and shows no sign of trouble while driving.i have no idea what it could be.any ideas??