Place potatoes in a large saucepan with enough water to cover, and bring to a boil over high heat. Reduce heat to medium low, cover and simmer until potatoes are tender. Drain and cool; cut into 1-inch cubes.
In a large bowl, combine potatoes with onion, celery, roasted red pepper, pepperoncini and eggs.
In a small bowl, mix mayonnaise, sour cream, pickle relish, Dijon and yellow mustards, garlic, dill, parsley and red pepper flakes. Pour mixture over potatoes and stir gently to combine. Season to taste with salt and pepper. Cover and chill for 2 hours before serving.
