door locks on 02 ford fiesta?

i have a fiesta sj02tlu pre 02 model. central loking on doors locks and then reopens on both doors this also happens when you lock the car from inside. apart from when you lock both doors ( pushing both leavers ) at the same time on the inside. i have sprayed with wd40 thinking it may be a dampness or watterm problem.but no effect. can you tell me the cause and cure to this prob. bill