Vauxhall Zafira Active 1600 2003?

when I have been out to shop and have left the car standing for about half an hour, I go back to it drive down the road and then all of a sudden the spanner comes on, it drives ok but the other day at traffic lights it lost power and I crept round the corner at the lights, turned it off waited a few minutes turned the ignition on and it started up, took it to the dealers they did a diagnostic they said they could not find anything wrong with the car they even road tested it and still nothing. I don't now what to do any ideas.