Permit me to walk myself through this situation. Perhaps it will spur some connection with someone else and then they can find the solution. Photocells usually fail so as to leave the lights ON. If it was a night-time problem it would be reflected light causing the blinking. Since it happens in the day, assuming its not in some real dark place, it should leave the lights turned off. When the lights are first turned on, the photocell takes some time to respond to daylight, and the lights are turned off. If there was a loose connection, it would cause the photocell to turn on the lights erratically, not "blinking". If you have only one fixture in the sign, then it could just be a problem with the fixture ballast overheating and tripping out thermally..over and over again. But the light wouldn't be on in the first place if the photocell was working ok. If it is multiple lights..then I'm stumped! Not likely a mis-application of photocells, since the sockets are keyed for the operating voltage, but if it is hardwired in, perhaps someone else replaced it with the wrong voltage rating, and everytime you replace it it gets toasted because of too high a voltage. That would leave the light turned on, and the resulting blinking might just be a ballast problem, as I mentioned earlier....