can someone please help me!

i have a '98 mondeo st24 which has developed a problem with the door locking (from the key)

when you unlock the drivers door, only the drivers door unlocks about 10-15 minutes all the other doors unlock.

Is this gonna be a problem with the drivers door lock/solinoid or is it more likely an electrical fault with the c/locking unit??

any help greatly appreciated
Dave