Sampling Rate and Fifo Rate

I'm experimenting with various fifo-rates in an attempt to get quaternions out of the fifo at rates > 100hz.

I'm using two mechanisms to control the fifo rate.
1. Setting the SMPLRT_DIV register (0x19), and
2. Calling the inv_set_fifo_rate() function.

With SMPLRT_DIV = 4, and inv_set_fifo_rate(1), I get good clean data at 100hz. If I tilt the IMU up and then back down, the quaternion settles rapidly in the expected position. All is good.

If I call inv_set_fifo_rate(0), regardless of what SMLPRT_DIV is, I get *very* noisy quaternions. Why?

If I set SMPLRT_DIV to anything other than 4, I observe a very slow settling time of the quaternions. In other words, if I tilt the IMU up, then bring it back down, it overshoots and then takes about 3 seconds to settle on the correct place. Why is this happening?