Vauxhall Tigra randomly immobilises engine?

2008 Tigra has a problem which seems totally random. The key unlocks car and turns over engine but the engine does not start and engine management light flashes. Then once I got it to the Dealer for inspection it worked again and they reported that no reports were found wrong with engine management!! So I drove it home and then next time I went to start it it had immoblised again.
Does anyone know if this is common and how the hell I fix it?? Both car keys have the same effect! What the garage should replace to render my car useful again??