Please help! White dots on surface of butter cake?

Hello, I'm a baking enthusiast who is hoping to sell my butter cake at farmers markets soon. I have never had any problem baking in my home oven, but the past few times I have practiced at a commercial kitchen I've had problems with the appearance of my butter cake. After a few hours, little white spots appear on the surface of the cupcakes. They get worse as time progresses. I have tried changing so many things but can't seem to solve the problem. I usually use organic sugar, so thought maybe it was the coarseness of the sugar not dissolving in the liquids which caused the spots so I warmed the milk and dissolved the sugar in it. Still spots appeared. I thought it may be the high heat of the commercial oven, so I baked it at a lower temperature and longer time..this made the dots worse. When I make this cake, i mix the wet and dry ingredients, then stir in melted butter. i haven't changed any ingredients or changed my mixing process, I've simply started baking at a commercial kitchen...the only thing I do differently is opening the oven during baking. The commercial oven is much stronger so usually my cupcakes start to rise toward the heat source, or the cupcakes along the edge get darker faster so I have to open their oven and turn the pans around..could this somehow cause spots?? I cannot figure this out and so cannot apply for markets anymore until I do. Please, any suggestions are appreciated!