In my recent training, I've been suffering from a similar affliction. I too have my right pointer finger and middle finger go numb while riding hard. This situation started when I began wearing my new pressure suit and I've noticed that the sleeves are somewhat tight on my forearms. This indicates to me that it's nothing more than a slightly different version of the arm pump that I've been struggling with for years. I found that pressing my hand against the throttle (to maintain my throttle position) and opening my fingers during a relatively smooth section of my training loop will cause the feeling to return, at least for a while.
The long term fix is commonly considered to be, by the magazine guys, simply improving your cardiovascular conditioning. The problem is that some people suffer from this more than others for no apparent reason. There was even a surgery developed years ago that "released" the muscles in the forearm from the capsule that constrains them. Several of the top motocrossers had this procedure done and it was rumored to help.
04 GO!!!!, 08 KTM 320 SXF, '07 Beta Rev 3,
Frankenberg 501, '96 Husaberg FC600, '68 Yamaha DT1, '74 Bultaco Alpina 250 (Mom's old bike), '84 Husqvarna WR400 - My Dad says I've never seen a motorcycle I didn't want to own...