I have the same problem with my light/speedo. My light is a Nite Rider 500 something, the computer is a Catseye wireless unit. What I have noticed is that if the light is on high or pulse the speedo works fine, low or medium causes the speedo to quit. I've come to the conclusion that it is a elector-magnetic field caused by the light causing the problem. Unlike a "nromal" light, LEDs are pulse width modulated to achieve "low light" settings which sets up the interference. As mentioned shielding the light with foil or such should stop it but it looks like kaka. The other option is to move the two as far apart as possible which worked for me.

fasthair