There was a thread about this very same issue in the bicycle gadgets sub-forum:

http://www.bikeforums.net/showthread...-cyclocomputer

There are a number of bicycle headlights out there which produce oscillating magnetic fields. These fields interfere with analog wireless cycle computers. If you have enough "real estate" on your handlebars, you might be able to put enough distance between the light and the computer to prevent this.

My original bicycle computer was a Bontrager Trip 5W analog wireless computer. When I installed a Planet Bike Blaze 2W headlight, my computer went haywire. I was not able to increase the distance between the light and the computer, so I solved the problem by buying a digital wireless computer, a Bontrager Node 1. Of course, switching to a wired cycle computer would also work.

Note: this is not just an after-dark issue. I use my headlight in flash mode almost every time I ride on the roads.