Yaw drift when using two servos

First off all i would like to thank you for the great work you have done. We are using Sparkfun 9DOM IMU and the freeIMU code works great. We get a very steady reading and when we use the processing code we get a very steady 3D cube.
But, when we are trying to use the getYPR function in order to stabilize a platform with two servos the roll servo works just fine but the yaw reading are drifting very strongly. I guess it has some connection to the magnetic flux generated by the servo motors. How can i get better readings in the Yaw angle? maybe something with the Mgmtr gain?
Shai