What is the best way to send fairly fast sign changing angular velocity commands to the 6470H driver board?

I use the ST EVAL 6470 driver board to control a stepper motor in my tracking system! I need to send out some fairly "fast" sign changing commands! This seems to make the control board unstable! The Busy/Sync led starts flashing and starts an unwanted oscillations! I try to read a busy status register from the control board to find out what would be the good time to send out a new command, but doesn't seem to be working! Any comment would be appreciated.