In a large pot of lightly salted boiling water, cook the potatoes, garlic, and bouillon until potatoes are tender but firm (about 15 minutes). Drain, reserving water.In a bowl, mash potatoes with sour cream and cream cheese, adding reserved water as needed to attain desired consistency.Transfer the potato mixture to a slow cooker, cover, and cook on Low for 2 to 3 hours.Just before serving, stir in butter and season with salt and pepper to taste. Sprinkle with parsley flakes if desired.