99 silverado 4.3 no power at idle, HELP!!!!

99 silverado 4.3 automatic 4x4 runs smooth, no codes, put it in gear, pulls the engine down to 5oo rpm, you have the brake on, and floor it the rpm goes up to about 900 and the trucks just acts like it is still idling. once you take off and the rpm gets above 1500 it has plenty of power and runs great, put new plugs in it checked the cat converters, could it be the torque converter??