first of all make sure your locking mechanism is not seizing up and is working ok, then i would suggest removing your speaker covers in the front footwells and checking the plug that connects to the central locking motor (think its the white/clear plug) these pretty much always corrode and play up, i just reomved mine a soldered the wires back together eliminating the plug, worked a treat and has been fine for over 3 years, may help you out.