Just discovered something this morning about the problem. I've been using 2D motion blur during the sequence. When the blur is on, the light turns off for those 15 frames (it turns off twice, both for 15 frame durations...). I turned off the 2D motion blur and.... the light came back on!

Looking into it right now...

Oh, and Dan... the camera actually doesn't intersect with the cone of volume light during the "blackouts".