I tested my oven and the temp is perfect. Could 400 be too hot?
400º seems hot for a cake. Most cakes bake at 325º to 350º. It would help to know what recipe you are using.
It's Louisa's cake, the one that won the ricotta contest.
it looks like you are not alone: http://www.food52.com/foodpickle...
I'm wondering if location in the oven, size of pan, and pan composition are the variables that affect it.
Might be the pan. Might be the oven.
Consider preheating to a little over 400 and turning it down to a bit below 400 when you put the cake in. That way you don't get the high direct heat of the oven trying to bring itself back up to temperature after the door's been opened and shut. You'll probably still have to leave it in longer than the recipe asks, but it might help with the over-browning.