Theft Deterrant system

Hello I am new to this. I hope someone can help me. I have a 2002 pontiac grand prix GT 3800 series with only 26,000 miles on it. When I use my keyless romote I hit the lock button once. then my car locks and my headlights flash but for some reason my car acts up every now and then My headlights will flash like every 30 seconds. I have tried unlocking the car then relocking it and it does the same thing. I have also locked the car then hit the lock button once on the keyless pad waited a few seconds then hit lock again to where my headlights flash and my horn will honk and it still does it. Like I said it does not happen all the time just once in awhile. If someone can please help me figure this one out it would be great. Thank you for you time