over heating problem

i have a mustang that is over heating and don't know what the problem is. i have a 3core radiator, mac pulleys, and a new thermostat. i just had the block rebuilt and it still heats. when i'm at 2200 rpms it cools down, but at lower rpms it just starts to heats up. i heard that a cobr water pump pulley would put a stop to it. but i'm not to sure. any info will help thanks