The grips expand as they get hot... they get longer. That expansion causes the grip to stick as the ends rub. You need to give the grip more room. Moving the assembly that holds the grip, the brake lever, the mirror, etc., in toward the center of the bike will give the grip room to expand.
This is usually exacerbated by the addition of a throttle lock at the outer end of the grip. As Moss found out it can happen to a stock grip, too.