occasionaly none starting range rover

Hello i have a range rover that will start well normally but occasionaly it wont start the lights come on except the glow plugs and the car ticks, everytime it does this i open the bonnet (and one time randomly) tapped the top of the ecu then it starts now i hav to do this wenever it doesn't start! i find this wierd and im hoping that someone has had this problem and maybe beable to direct me
Thanks

try spraying the contacts with wd40 then leave to dry before reconecting