General Recipe Trouble Shooting

Hi!
I use italian meringue for toping lemon tart, and for baked alaska desert in my restaurant. I have no problem with it when its served in 1-2 hours. When I do it ahead, a few hours before serving, and keep the meringue in fridge it leaks a bit and becomes porous/ the tiny bubbles become much larger and it is no longer a light mouse/marshmallow texture I like. Do I overbeat it or is my syrup not hot enough? What is a proper technique? I know people store it in a fridge for hours with good result.