Try shorting the contacts in the sensor. The easiest way to do this is to take the battery out, turn it over and put it in for a few seconds, then take it out and put it back in properly. Shorting the contacts forces the sensor to reset.
Then the only other thing I can think of is to try another new battery. The only time I've seen one of my speed sensors do the continual flashing thing was when the battery was low, and just putting a new battery in fixed it. Maybe the battery is a dud.
