Hmm... this is only my bit of advice but worth a try... might be worth getting a good biomechanical assessment done by a physio or podiatrist with video analysis equipment... this would involve them using a high speed camera to slow down and analyse the way you walk and run... very often it can be the slightest bit of pronation or supination (low or high arches) that can repeatedly sprain (or prevent healing) the ankle ligaments slightly as you walk. This along with just examining you up front might uncover the problem? This is just one possibility of a cause but regardless they might well uncover other causes. Worth a try