2000 Mustang has Oil in the Water

I have a 2000 3.8L v-6 stock Mustang that has oil in the water. She does not have water in the oil though. She had the heads rebuilt about 12000 miles ago, a new thermostat put in at the same time the heads were rebuilt and a new radiator put on about a month ago. We flushed the system with water, removed the hoses and cleaned them inside and out, removed the resevoir and cleaned it inside and out and put in fresh coolant. The new radiator worked great for about a week or so and then oil started showing up in the water again. She runs great. She stays cool to normal. She is not leaking oil or water onto the ground or around the heads. No one seems to know exactly where to go from here. Any suggestions or ideas?