This is true for just about all wireless components and HID lighting systems that try to co-exist on a handlebar.

The solution for me was mounting my light on my helmet; no more interference. Morever, I discovered that a helmet-mounted light does everything it does well off-road when you're also riding on-road. I sometimes wish motorcycle helmets could legally be used on public roads with an "adaptive" lighting system; it's sure nice to be able to see what I"m looking at when I'm riding my bicycles in the dark, e.g., through corners, off-to the side of the road, down at my computer and bike, etc...

P.S. Thanks for all the tips on GPS. I was sort of joking about that. I'm good with what I have. If and when the HAC4 and Polar die, perhaps a GPS will be of interest. In the mean time, I've got MapQuest and a road atlas for my car and there's almost always someone on big bike and tandem outings with a GPS these days who I can follow around if need be.