It sounds to me as though you have got a syphoning problem with your sink, the trap is being emptied by another sink or bath discharging down the system, remove the trap, take it to a builders merchant, do the girly act and tell them you need an anti syphon trap, when you refit it remember to use fairy liquid around all the rubber bits on the connections and do not do it up too tight as it will distort the seals and make it leak.
Derek
June 2004