Check Engine Light in Corsa, dangerous?

I have a 1997 Opel Corsa. Last Oct 2007, the car broke down. Turns out the engine was f**ked. Got a replacement engine a few weeks ago, passed it NCT then 2 days later the exhaust broke. Got that repaired. So was bringing the car down from donegal to Dublin. 3 hours into the journey, I stop at traffic lights. The engine light comes on. I drive to my destination, turn it off, back on again and the light goes. Next day drive to the shop. No light comes on at first, but when I stop in traffic, the light comes on again. I replaced my oil but it still happens. Any ideas? It's only when I stop in traffic. and it comes on.

Is it dangerous for me to further drive or what?