I have had this problem for several years now and have not been able to find a solution. My heel on the left foot slips up and down constantly when i am running while my other heel is perfectly fine. I have tried many different socks/shoes etc. and nothing has worked to solve this. Any advice on what could be causing this problem and what can i do to prevent this from happening. I am a college runner with a 14:25 5k and have struggled with this issue for a while. Any help is appreciated