Put the potatoes in a large saucepan, cover with water and bring to a boil. Simmer the potatoes over moderate heat until tender, about 30 minutes. Drain and let cool. Peel the potatoes and cut them into 1-inch cubes.
In a large bowl, whisk the mayonnaise with the sour cream and mustard. Add the celery, parsley, chives and tarragon and season with salt and pepper. Fold in the potatoes, mashing lightly. Refrigerate until chilled before serving.