afremont:
The stall current of the motors is such that they would overheat the controller pretty fast, but I assume that you aren't just turning them on at 100% duty cycle from a stand-still, are you? What PWM frequency are you using, 10kHz is pretty high. Depending upon your motors, they might have too much inductance to actually draw much power at that frequency.
I'm not sure what you mean, I'm using the arduino PWM 155/255 so .58 duty ratio.