[Kevin Sevcik]

Are you sure the motors aren't just topping out at full speed? That can happen if you do something screwy and your command value doesn't create a change in feed back in the same direction. For instance, a 0.5 command to a speed controller should give you a positive speed feedback. If a 0.5 speed command gives you a negative feedback, the the motor is going to quickly end up going full speed the wrong way.

So, if the motors are actually just running full speed in a direction, then something's backwards with your speed feedback or motor polarity from the speed controller.