it may be the margarine -- whenever i used to make frosting with margarine, it would eventually start separating out. dunno if anyone else has had such a problem, though. if things get too runny, just add more powdered sugar. also, when i make frosting, i never let it melt completely in the microwave -- you're aiming for soft or room temperature, NOT melted. trust me, temperature of butter in a recipe makes a huuuge difference when baking. regardless, refridgerating it will almost always help it stiffen up in the fridge.
