I am using the Euler angle output, but in certain orientations my yaw and roll values begin to look strange. Why is this happening?

If the pitch value nears 90º, this may result in inaccurate readings for the yaw and roll angles. This is due to gimbal lock, which is an inherent limitation with Euler angles. You can read more about this here: http://en.wikipedia.org/wiki/Gimbal_lock.