Take out your ignition rotor. Destroy it and buy the red uprated one from Moss. The black ignition rotors of late have a rivet that causes a path to ground to the shaft when it gets warm. I had this exact same problem on my Bs. New red rotors fixed it for good. It could be something else (no venting in fuel tank, etc). But get a good baseline and get a good rotor. Check your plugs, cap, wires (or replace them, not overly expensive, if you haven't done them in a while).
Paul Seeberg