When your brushless motor tries to draw more voltage than the battery and/or speed control can provide. This results in your car or truck sputtering (lurch forward, stop, then repeat in a short period of time) and is usually due to thin wire gauge used on the speed control, battery, and/or motor. A brushless motor requires a lot of power, and needs it quickly. But small-gauge wires can only provide so much. It is similar to a garden hose. You can't put out a fire with a hose the width of fuel tubing. But before you go to get your soldering iron, recalibrate and reprogram your speed control. If that doesn't solve your problem, then consider upgrading to a thicker wire. You should also use Deans Ultra Plugs on your batteries, speed control, and charger to prevent cogging, and experience longer runtimes and possibly higher speeds.