Renualt Traffic 53plate alarm problems?

Hi, I have this problem with my 53plate traffic, the central locking should normally lock with one press of the key, then two to dead lock it, then if you keep the lock button pressed down the alarm sets. But mine decides to set itself after the 1st press, but only sometimes. The alarm will sound for no apparent reason within a minute of locking it. Very frustrating! any help please? (this is a standard factory fitted alarm)