I usually bake cupcakes at 350 so you may want to try that. For the higher temperature, start checking for doneness around 17 minutes. If that’s not the issue, it may also be that leavening is off? If they cakes rise high in the oven, then sink, it may be too much leavening that’s then collapsing a bit. Usually you need about 1 1/4 teaspoons baking powder OR about 1/3 teaspoon of baking soda per cup of flour in your recipe. Whether you use baking powder/soda depends on the recipe and how many acidic ingredients there are (such as buttermilk, cocoa, lemon juice, etc.). Good luck!