my vauxhall corsa 02 reg dashboard display keeps going off?

my vauxhall corsa display keeps going off if the car is already running the indicators then don't work if not started the car the immobilser kicks in causing the car not to start if the top of the dashboard is tapped quite heavily it will then restore all functions aparemtly loose connection behind dashboard been repaired by aa once and a mechanic but connection keeps being lost why and how do i permanently fix it