If the frequency is fairly rapid, then I'd say you are getting light reflected into the sensor. Correct me if I'm wrong, but don't mercury vapor lights take a few minutes to cycle on and retain some glow for a few minutes after they are off? If so then "fairly rapid" in this case could be 5-10 minutes.
Try a simple test. Block the sensor so it thinks its night time. If the light stays on, then you are getting light into the sensor. If it continues to blink, then at least you've eliminated reflected light as the cause.