Fiat Punto start up

On start up 50% of the time the dash lights go out, the engine still cranks but doesn't start, after 2 or 3 tries it fires up and runs fine, the lights don't go out on cranking when it starts. The problems the same on hot or cold starts. I checked and cleaned the earth lead from battery to inner wing and gearbox, still the same problem. the battery seems fine, the clock still displays the correct time after the lights goes out on the dash.
Please help!