MY DASHBOARD LIGHTS/INSTRUMENTS DON'T WORK. WHAT IS WRONG!?

I have a 2005 vectra c and occasionaly when I start it up the dash lights/ instumemts do not function at all, everything else works fine, however after approx 5mins of running it all lights up and everything works OK. This only seems to happen when the weather is cold or when the car has been stood a few days. Any ideas as to the cause? any help would be appreciated