Southern Style Mustard BBQ Chicken Kabobs

1 cup prepared mustard

1/2 to 2/3 cup cider vinegar

1/2 cup Equal Spoonful*

1 tablespoon Worcestershire sauce

1/2 teaspoon maple extract

1/2 teaspoon coarsely ground black pepper

2 tablespoons stick butter or margarine

1 1/2 pounds skinless, boneless chicken breasts, cut into 3/4-inch cubes

2 small yellow summer squash, cut crosswise into 1-inch cubes

12 medium mushroom caps

1 large green pepper, cut into 1-inch pieces