I recently read an advice letter about the same thing and they said to check the position of your shoulders while you ride. Apparently where your brachial plexus (the nerves that supply everything to your arms) exits the base of the neck through the scalene muscles, can be affected by shoulder position. If you squash these nerves before they even get to your arms it can cause numbness. To relieve it we often sit up a bit and shake the hands around so we think its the hand position but it is often at the shoulder/neck. You wont be getting pain in the neck though as its just the nerves passing through the scalenes.
I get numb fingers on my upright commuter and had noticed it was more in the head-wind and when I was pushing it harder. My instinct is to hunch up my shoulders when I ride hard. I now make a conscious effort to relax my shoulders, even to pull them down, towards my waist and it has really improved.
Worth a try?