When I ride my bike underneath low slung power lines (large National Grid cables) nearby I get a shock from the handlebars. This only seems to happen if I'm moving.

I connected a digital voltmeter which recorded values up to and off the scale of its 600 volt limit (I have a short video of this). The other peculiar aspect (which I noticed on the video) was the the bike speedometer showed fluctuations from about 8 mph (around the true speed) to over 30. Any ideas about why these things should happen?