The head gaskets blew in my legacy so a friend rebuilt the motor.  I had the heads machined,all new gaskets, plugs, wires, filters, knock sensor, water pump, timing belts and all pulleys, t stat and radiator.  I swapped from auto to manual and did a lift.  The started right away and has never ran/idled better.  When I drive, the motor will hit a certain rpm range and hit a flat spot where it hesitates and doesnt want to go.  If you push the gas harder it will hesitate and then power through it.  Its intermittent and doesnt always do it.  Any thoughts on what is causing this?  Thanks!!!!

Throttle position sensor?