2002 Jeep Liberty engine idle

I bought a 2002 jeep liberty sport with a blown motor from a friend(some friend). I had a local mechanic install a new motor in the car, plus it needed a new intake and new oil pan. The car runs and drives great, the problem is that it will not idle properly and stalls frequently at idle, it always starts right back up. The mechanic has put in a new battery, throttle sensors, knock sensors, iac valve, computer and reflashed the computer. He checked out the catalytic converter. We are thinking maybe a wiring harness problem? The car runs fine but when u come to a stop the motor fluctuates between 500 and 1000 rpm and finally stalls. Any ideas?