WA Audi A8

A couple of weeks ago I noticed that my 1998 Audi A8's alarm started acting funny; when I'd lock the car, the beep slowly got quieter until it stopped beeping and the lights stopped flashing, the vacuum pump also seems to stay on a bit longer when I unlock my doors. The lights still flash when I lock my doors and the alarm indicator light on the dash still flashes. I've read in a few places where people have replaced the central locking module or the alarm module or both at the same time. Has anyone out there had this problem before?