Replacing the central lock motor/solenoid will cure the door issue. However, you may want to check that is just isn't a loose connection or fouling of the mechanism.
Also I was under the impression that although you have "locked & alarmed" the car, there is a delay which stops the immoboliser and siren function until the turbo timer has completed the count down. If the alarm siren was to arm the moment you locked the doors then technically the alarm would sound because your motor is still running and the sensors would be triggered.
My M80 beeps as I lock it and then about 10 secs after the turbo timer has turned off which is the immoboliser and siren arming.