Have you driven it much? Have you done the idle relearn? Which tuner did you go with? Does the engine idle fine otherwise? What does it do under normal driving?
The O2 sensors are farther away from the engine than their original locations and there are some O2 transfer function tables that can be adjusted to account for the increased distance. Some tuners make this change and some don't but my understanding is that the ECM will eventually learn the correct values on its own.
My guess is that it's learning fuel trims. If the engine runs fine otherwise then drive it for a while and see if it evens out.