Dredge chicken in flour; remove excess. In large heavy non-stick skillet, heat oil over medium high heat. Cook chicken on each side for 2-3 minutes, or until golden. Add stock, vinegar and pepper. Reduce heat to medium-low, cover and cook 5-10 minutes or until chicken is tender. Remove chicken from skillet and keep warm. In small bowl, dissolve cornstarch in water, stirring until smooth. Add to skillet and cook 1-2 minutes, stirring, until thick and smooth. Pour over chicken and serve.