Well after the feedback here and talking to the guy that built my trans, I moved the sensor to the line leading back to the trans (post both radiator and external cooler). That reading seems pretty steady and acceptable, within about 10 degrees of the engine temp. I ran just the external for a morning, and didn't like the temps I saw sitting in traffic with minimal airflow. Hooked both coolers back up, and it seems much better.

Thanks for the feedback!

-Dave