Yaw Control

I have some really crazy yaw control. The yaw seems to just drop off over time? What is going wrong? I have a TBS discovery quad that has a Pixhawk on it. The roll and pitch seem stable but the Yaw is crazy. When I try to fly the quad yaws uncontrollably. But the pitch and roll seen really stable. The dropping yaw can be seen in the attached picture. I have adjusted the Yaw PID’s quite a bit but why is the Yaw moving like this? Shouldn’t it be stable when it is sitting on my desk?