2006 Vauxhall Zafira boot problem clicks open by itself

hi iv got an annoying problem with my zafira, it seems every time i start the car the boot lid pops open the light and beeper comes on to let me know it has opened, when i go out and close it it seems to lock ok but it can then open again at any time without warning even while i am driving the car any advice would be really appreciated with this as getting a bit worried about taking the kids out in it now thanks.