Or it could be a pinched nerve in your spine. I had to have a couple of vertabrae fuzed together a few years back, and my right foot does the same thing. On nice, hot days it doesn't bother me (bad deal since I live in AK), but if the temperature drops even just a little, the numbness comes on. If it's not too cold, I can curl and uncurl my toes a few times and it brings back the feeling. If that doesn't work, stopping and walking around does the trick. If it happens when racing you just have to deal with it. I'm not sure you're having the same problem, but this is what I do.