Combine all ingredients except grape clusters in a large metal mixing bowl; mix well. Freeze until ice crystals form around upper edge of mixture. Remove from freezer, and beat well with an electric mixer. Return mixture to freezer. Repeat freezing procedure until mixture reaches the consistency of a sherbet. Spoon mixture into a 6 - cup ice cream mold, and freeze overnight.
Unmold and garnish with grape clusters, if desired. Scoop into sherbet dishes, and serve immediately.