I have an 05' g35 sedan. I have only noticed the problem on the freeway after driving for 10 or so mins, sometimes longer. I have noticed the brake light will illum. and if I run over the center line reflectors or hit a freeway expansion joint it will flicker or sometimes go off. I haven't looked to much into the issue. No other lights are illum. just the brake light. Brake fluid is filled to spec. & the harness from the brake master cylinder level sensor doesn't look distressed. Maybe the level sensor is shorting & reacting to road bumps...? But what would freeway speeds have to do with the equation? I don't believe it is the park brake sensor either because it works fine when brake is applied & released. Anything else temp. or speed related that could cause the brake light to illum.?