So dumb... but stirrups slipping?

You say you keep your heels down, but you also have to keep your weight in your heels. If you are balancing off of your thighs, knees, or calves, then that will be why. You may not notice it. Next time you ride, think about keeping your weight in your heels. You can exaggerate it at first. Try to ride without touching your knees to the saddle, it will make you keep your weight in your heels.