Hey pajiko, Well considering ur complains it seems you gotta a problem in your tibio fibular joint. as you said, externally rotated foot doesnt cause a slip and an internally rotated foot does, it means that it is your tibilfibular joint actually that is subluxating and repositioning. Also the biceps femoris muscle of the hamstring muscle is inserted on the tibiofibular joint after inserting on the lateral condyle of the posterolateral part of femur. So it does suggest that sliping of the tendon is due to actually moving of the tibiofibular joint. I advise you to go to a physio and ask for taping your tibiofibualar joint in a position when ur foot is in external rotation. also go for Ankle strengthening as superior tib-fib joint is directly linked with inferior tib-fib joint and in future may cause weakness in ur ligaments and muscles of ur ankle making ur ankle prone for injuries.