Hi All - I have had the same problem for the past 10 years and in the last two months I've finally managed to solve it by fore-foot running. I damaged my ankle running over some rough ground - if I remember rightly it was my deltoid ligament. The doctors said that the sheath around the ligament becomes inflamed after running for a certain amount of time (in my case it was around 5k), when it swells it stops the circulation to your foot and then eventually your lower leg.
Naturally I land on my heel and roll off the front of my foot - my girlfriend suggested that I should try landing on the ball of my foot rather than my heel. Until I tried this I would struggle to run further than 10k because my leg would become completely numb (it feels like a dead stump when it happens) - I've just got back from a 13 mile run with absolutely no problems - seriously I'm over the moon! I hope this technique works for some of you guys, I know how frustrating (and expensive!) the problem can be.