Peugeot Partner 1.9d 2002 Central locking (berlingo)?

Central locking keeps playing up. Sometimes it 'bounces' (Locks then unlocks straight away). Other times it locks fine then unlocks itself at some point before my return. Seems to happen mainly when the weather is hot. Been to pug dealer, and guess what, it worked the whole time they looked at it. They say they can't fix it because it is an intermitant fault. Any ideas?