I've recently noticed this in a couple of races and workouts. It doesn't happen very often, but sometimes (only during a race or a hard long interval/time trial), my lower arms and hands get extremely cold and stiff/numb. They feel heavy, too, and it's hard to get into any kind of rhythm and I can't hold the pace I should. It's not a weather thing, either; it's happened in different temperatures.
Anyone ever experience this or know what causes it or how to prevent it?