It might be your front brake light switch. On my FatBob the spring on it was weak and wind pressure on the front brake lever at speed would cause it to make contact, which the bike interpreted as being ridden with the brake on - which in turn made the light illuminate at speed, but not when slow or stationary.

It's a known issue with Fat Bobs, but I don't know about other bikes....

 

Problem solved!!

 

My switch does stick sometimes