Are you using a schottky diode? If so, get it off there. I assume you don't have a schottky on there, but I had to mention it.

Next, check the timing on your motors. If I were you, I would keep them at a maximum of 10 degrees. You have to consider that in reverse, your timing is reversed, and that 20 degrees of timing then becomes 20 degrees of retarded timing, and will make wierd crap happen.