Stir the stock, soy sauce, vinegar, sugar, garlic powder and red pepper in a shallow nonmetallic dish or a gallon-size resealable plastic bag. Add the chicken and turn to coat. Cover the dish or seal the bag and refrigerate for 1 hour. Remove the chicken from the marinade.
Lightly oil the grill rack and heat the grill to medium. Grill the chicken for 15 minutes or until it's cooked through, turning and brushing often with the marinade.
Heat the remaining marinade in a 1-quart saucepan over medium-high heat to a boil and serve with the chicken.